TLDR: More teeth per inch (TPI) = smoother cut. Less TPI = faster cut.
Now the long version. The dovetail saw is smaller in all ways and has more teeth per inch with a ripping profile. As the name suggests they are designed for doing dovetails and I doubt the actual joint will often make the 40mm depth an issue, which is another reason why the saw is smaller, you wont be asking it to cut huge amounts of stock in one go so you might as well take a few extra strokes and have a surface which needs little to no work to be ready to join - this is of course if you're into the whole 'perfect' dovetails concept or just want a functional joint. The saw is shorter as well, presumably for more control but once you get used to a saw I'd question that theory. The carcass saw is sized between a dovetail saw and a tenon saw and is intended to be used for cutting the timber for making the carcass of traditional cabinets. There is actually a rip cut and crosscut version of these saws with the difference being the number and profile of the teeth. They both will have less teeth than a dovetail saw so will be rougher but will complete the cut quicker, with the crosscut being the fastest of the lot.
My recommendation is simple - if you can spring the budget for 2 saws - get the crosscut carcass and the dovetail saw. If you can only get one, maybe consider the rip carcass saw. It will crosscut small timber fairly easy and leave an okay-ish finish for dovetails. Option 2 is buy the dovetail now for it's intended purpose and then if you find you want the crosscut further down the line to do carcass work, no problem. I personally cut most of my dovetails to be strong functional joints so I'm not too bothered about having completely gapless joints. You'll find that to be a feature that only woodworkers look for whereas your average person really doesn't care or notice. Like all small mistakes we make, we are most commonly our own worst critics. Don't point it out to anyone and it will never be noticed.
