If you have some old manual focus Pentax lenses, they can easily be adapted for use on modern Canon DSLRs (so can vintage Nikon, Olympus, Leica, and others:
http://bobatkins.com/photography/eosfaq/manual_focus_EOS.html). Sony also can accommodate many other manufacturers' vintage lenses.
Aside from Nikkors, most cannot be easily adapted for use on Nikon. The same is true of the other major brands.... Pentax can use Pentax, Oly can use Oly, etc. But there's a lot less adaptability across brands, than there is with Canon or Sony.
The problem with asking a question like this on a forum is that it brings out each brands fanboyz and fangirlz, who will rabidly defend the brand they chose and own.
This is an example of that...
Problem is, D750 is a full frame model, which is something the original poster already ruled out.
The fact is, D750 is a great camera and either Canon or Nikon can do a great job.... So can Pentax, Olympus, Sony (formerly Minolta, except for a few years when it was Konica-Minolta).
Canon and Nikon have the largest selection of cameras and lenses. In a lot of ways, the lenses are more important than the cameras they're used upon. So be sure to compare the lenses offered, taking into account what you might want now or in the future.
Canon makes their own imaging sensors. So does Sony. Everyone else buys their sensors from Sony or other sources. (Canon recently changed policies and announced they'll be selling their sensors to other manufacturers, too... But I don't know of any deals yet.)
Canon pioneered multi-point autofocus systems, image stabilization, CMOS sensors, extensive use of Fluorite lens elements, full frame DSLRs and to a large extent video in DSLRs.
But Nikon has a long, deep history of photographic innovation, quality and performance too.
The more affordable Nikon DX (APS-C) cameras - D3000-series and D5000-series currently - require AF-S or AF-P lenses to be able to autofocus. In order to autofocus AF-D lenses, a D7000-series or higher Nikon model is needed (with an autofocus drive motor in the camera body, because those lenses do not have built-in focusing motor). In contrast, all current Canon APS-C/crop sensor cameras are fully compatible with all Canon EF and EF-S lenses manufactured for the past 25+ years (10D from 2004 and earlier models cannot use EF-S lenses... but you aren't likely looking at 12-13 year old camera models).
At their base ISO Nikon and Sony have the widest dynamic range sensors... about one stop wider than Canon. As ISO is increased the difference in dynamic range decreases. At about ISO 800 or 1600 they're pretty much the same. Beyond that at higher ISO settings Canon have slightly wider dynamic range.
Third party lens and accessory manufacturers nearly always make products compatible with Canon and Nikon.... For Sony now, too, to some degree. A bit less for Pentax. And considerably less for Olympus, due to their use of the Four/Thirds sensor format (Olympus are able to use some other manufacturers' 4/3 lenses and accessories, that aren't usable on any of the other major manufacturers APS-C and full frame cameras.)
You're probably wise to look at crop sensor cameras. They're all most people really need. Full frame cameras cost more and limit you a bit, to using full frame capable lenses which also tend to be larger, heavier and cost more. Crop sensor cameras can use both crop-only lenses and full frame lenses. Plus, a full-frame telephoto lens on a crop sensor camera is "more powerful" than the same lens on a FF camera. OTOH, a FF wide angle lens is no longer wide, when it's used on a crop sensor camera. But there are lots of ultrawide crop-only lenses available to solve that problem.
Again, look beyond the camera itself, at the lenses and accessories you may want to use with it now or in the future. For example, let's say you'd like to have a truly wide lens for use on a crop sensor camera. There aren't many primes, mostly just zooms. These are available for all the camera manufacturers... but look more closely.
OEM...
Canon: EF-S 10-18mm STM IS ($280) and EF-S 10-22mm USM ($600)
Nikon: AF-S 10-24mm ED DX G ($900) and AF-S 12-24mm f/4 ED DX G ($1150)
Sony: 11-18mm DT ($750)
Pentax: 10-17mm SMCP-DA ED ($450), 12-24mm f/4 SMCP-DA ED AL ($700)
Olympus (Four/Thirds): 7-14mm f4 ED ($1600), 11-22mm ($700), 9-18mm ED ($600)
Third party...
Sigma to fit all brands except Oly: 10-20mm EX DC HSM f3.5 ($450), 8-16mm DC HSM ($700)
Tamron for all brands except Oly: 10-24mm SP Di II ($500)
Tokina for Nikon & Canon: 12-24mm f/4 AT-X DX ($400), 11-20mm f/2.8 AT-X DX ($500), 10-17mm AT-X 107 DX ($475-$530).
Tokina still offers a recently discontinued model for Nikon, Canon, Sony: 11-16mm f/2.8 AT-X DX ($430 to $480).
Wading through all the above can be confusing. You should look for and read reviews about particular items before making a decision. Interestingly, Nikon fanboy Ken Rockwell says the under $300 Canon EF-S 10-18mm is "Not only is it the world's least expensive, it's super light weight, and is also optically superb. There is no sharper ultrawide lens for APS-C cameras..." Ken failed to note that it's also the only lens in the category with image stabilization. Canon fanboy Brian Carnathan at The-Digital-Picture.com is a lot tougher on the lens, noting that it's a bit plasticky and has some slight shortcomings, but also seems very impressed by it.
You can do similar brand-to-brand comparisons with telephoto zooms or macro lenses or any other type of lens that might interest you. You won't find much difference in price and performance among "kit" lenses and some of the most common entry-level models. It's when you look beyond those to more specialized lenses and accessories that you may find some brands offer more choice and/or better value. The lenses and accessories you want now or in the future can really help narrow things down or steer you toward the best system for your purposes.
All the modern camera systems are excellent and highly capable. Sony is the "new kid on the block", bought into the DSLR market when they acquired Konica-Minolta in 2006 and has been doing some very innovative things with mirrorless cameras, electronic viewfinders and shutters, partnering with Zeiss on a lot of things.... but tends to be high priced with their lenses and doesn't yet have nearly the system selection of Canon or Nikon. Nikon has used the same F-mount since 1959 and there's quite a bit of backward compatibility.... but there also are a lot of incompatibilities or limitations among all the variations they've offered over the years. Canon made a clean break around 1990 to their EF/EOS system with it's electronic lens mount and has been at the forefront of DSLR and lens innovation for much of the time, but their system is not very backward compatible to their earlier FL/FD and R systems. Pentax is now owned by Ricoh and has emphasized durability and weather sealing, recently introduced a full frame model that their fans have wanted for about ten years, and also make some pro-quality medium format digital cameras (as do Fuji, Hasselblad and Mamiya). Thanks to the somewhat unique and smaller Four/Thirds sensor, Olympus is able to make their system smaller and lighter than most. Leica continues to be quite slow to adopt some of the new technologies and focus on the extreme high-end of the market with their offerings.
To a large extent, the playing field has leveled out a lot in the last few years. Everyone is now offering some form of image stabilization. Everyone is now using CMOS sensors. All the autofocus systems are similarly capable. Computer designed and manufactured optics are better than any time in history. Most manufacturers' DSLRs offer video capabilities and self-cleaning sensors. Other "gee whiz" features such as WiFi connectivity, GPS and more.... some of which might have little to do with taking photographs.... are relatively common, too.
Hopefully this will help you with your decisions.