It depends upon what he shoots most and how you define "best".

Canon's got a BIG discount on their M5 right now: $579 (regularly $979). It's the top-of-the-line M-series model with a 24MP APS-C sensor. That's more resolution than the full frame 6D DSLR. It has a better AF system and faster frame rate than the 6D, too. It's also got a slightly larger, higher resolution articulated rear LCD screen.

"Best"might be that it can share lenses and accessories with his DSLR, as well as have some familiarity since it's another Canon model. And it's significantly smaller and about half the weight of the 6D.

But there's no "free lunch".... There are limited M-series lenses available. To date Canon has only offered eight (in over five years developing the series). But if one or more of those work for him, it also will be smaller and lighter than the full frame lenses needed for the 6D. The M5 might also use smaller, lighter, less expensive EF-S lenses with an adapter (Canon's own EF/EF-S to EF-M adapter is expensive.... there are considerably cheaper Fotodiox, Vello and other third party that do the same thing). M5 in kit with an EF-M 15-55mm IS STM lens currently is selling for $700 (regularly $1100). Or it's offered in kit with an upgrade EF-M 18-150mm IS STM for $900 (reg. $1485).

Another consideration is that the M5 uses a much smaller LP-E17 battery. That and it's electronic viewfinder make for fewer shots per charge. It's rated to be able to do about 300 shots, compared to around 1100 with the 6D. Both cameras might be able to take more with some simple power saving tricks. And it's a bit unfair that the M5's battery performance is evaluated with 50% usage of the built-in flash (CIPA standard), which is a big drain on the battery. The 6D doesn't have a built-in flash and it's optical viewfinder doesn't put the same constant drain on the battery during use. But with M5, better carry one or two spare batteries if planning more than 300 or 400 shots.... If planning to shoot about as many shots as are possible with 6D.

The Canon M5 may not be the most advanced mirrorless out there... but it has been a real game changer for Canon as their first model with a built-in viewfinder. Since Canon introduced it, they have achieved 1st place ranking selling mirrorless cameras in Japan (this, of course, considers all models they offer... not just the M5). And it's part of why they continue to dominate worldwide in sales of interchangeable lenses of all type (both DSLRs and mirrorless), with double the market share of Nikon and four times that of third place Sony. Earlier this year Canon had almost 50% share, Nikon about 25% and Sony roughly 13%. All the other manufacturers divide up the remaining 12 or 13% of market share. It will be interesting to see what movement there is after this Fall's introduction of full frame mirrorless models from both Nikon and Canon.

When I see huge discount like this being offered, I suspect that there's a new model coming soon and they are clearing the shelves. If that's the case, the new model will no doubt be "full price" and the M5 will still work just as well as it does now! I recommend you check it out. There are lots of good reviews online, but here's one to start: https://www.the-digital-picture.com/Reviews/Canon-EOS-M5.aspx

Note: Canon's M50 mirrorless is also on sale, though not as deeply discounted as the M5. The M50 is very similar in many respects, including a built-in electronic viewfinder... but bit more designed for automation, with fewer controls for direct access to functions that more advanced photographers usually like to set for themselves. The M50 is a newer model, is one of the first from Canon using the new Digic 8 processor and priducing the new CR3 RAW file format. This may not be compatible with some older software... updates or upgrades of post-processing software may be necessary.

Alan - there's a French expression "chacun à son gout" - leave you and others to discover the translation - but it personifies your posts in spades !

A long-time photographer ? Not sure, but sounds like. Well-meaning in your responses? I'll give you the benefit of the doubt. Canon fan boy? Seems so, but many are ! Over-verbose in your "helpful" responses? Definitely .... to me at least ! Looking at your gear list I see no mirrorless camera, and would also mention that others here have questioned your mirrorless "hands-on" experience.

To the OP ...... please do your own research about mirrorless. I'm certain you want the best you can currently afford for YOU.

It's up to you to check out how Canon compares currently in the mirrorless stakes.

Hey, it may still be for you ...... !

Q

Regardless of whether Alan owns/shoots a mirrorless, I think if you review his responses over the years, you will find that he is one of the most knowledgeable and helpful posters on this forum. His responses may seem over-verbose to you, but I would instead describe them as thorough and comprehensive. Certainly he is a Canon shooter, but I have repeatedly found well-balanced reviews of other systems in his posts. I am waiting for you to post some useful information for the OP (as opposed to a critique of the current responses).