When I started in digital there were many cards but the two most popular were the CF and the SD cards. The divide was simple too, the professional gear used CF and the consumer gear used SD. The problem though, when you have two standards for the same thing, is that you end up with higher costs making and maintaining two different flavours of memory cards. So a decision was taken somewhere in the industry that the SD card would be the standard. As a result you'll find most laptops come with an SD card reader built in and you'll find that practically everything that's made now uses that card.

In terms of your questions, manufacturers who make high end stuff find themselves in a quandary. Those who use CF cards often have a large number of them. They would not appreciate being forced into buying all new cards and changing standards after spending thousands of dollars for a new high end camera. So many compromise offer a slot for each in the newer models. Eventually though I think you'll find that the CF card would disappear soon enough.

As to "better", both have issues. The CF card is quite robust and for a time had higher capacities and performance since it is also larger. The main issue with these cards is the many pins. If you accidentally bent a pin in the camera you were into an expensive repair.

SD cards seem more flimsy and delicate and until recently were limited in terms of how much memory they would carry or how fast it will perform. At the same time they don't have as many pins. They also have a switch that allows you to turn off the card thereby preventing write operations.

At this stage too it doesn't matter which is better, the CF card is on its way out so you may as well stock up on SD cards instead. In terms of performance, that depends on the camera. I use 60 MB/second cards for my D800 and my D3, that seems fast enough to allow the cameras to work flat out. If I ever have a faster camera I'll probably be looking for another faster card.

In terms of brand, the Sandisk and the Kingston ones have good reputations but I think you'll find the members here have many more favourites that they swear by. In fact it's hard to make a recommendation.

There are very few companies (two or three I think) who manufacture these memories and the controller that allows you to read and write to the memory. These two components are then assembled by the companies who place their name on the card. The problem is that the cards can have memory from one maker and a controller from another and this mix changes constantly, so in fact it's not easy to say that a particular brand is better than another because they all seem to come out of the same pot.

Regarding your choice of camera, I personally prefer FX sensors, they create higher image quality. That leads you to the D800 and the D750. At this point then how much resolution do you need?

I use a D800 because I do mostly macro work. To me that resolution is an answer to my prayers. Sometimes it's impossible to get close enough to a subject to fill the sensor. In those moments there's enough resolution to get a sharp crop. The down side of the camera is that it's slow, four to five frames per second, and it's unforgiving. With that resolution you better keep your lenses squeaky clean because every spec shows up on your images. You need to also keep the sensor clean.

For non macro work I prefer my D3, it's faster and allows me to work in very low light situations.

The same could be considered for your choices. I'd say the D750 is an excellent general camera whereas the D800 is more into areas where you need high resolution.