85 mm
I love shooting with primes as well, and after using (and loving) the 35mm/1.8 in tandem with the 50mm/1.8D as my two primary primes, I decided that it might make more sense to use the 35mm and an 85mm/1.8 to give me a little more range, or at least more spacing in between the two. I picked up a used copy of the 85/1.8D to that end.
In practice, and for the things that it turns out that I shoot, the 85mm on a crop body like my D7000 is actually a little too long for most of the places that I tend to shoot, but too short to really replace any kind of zoom. I was pretty surprised to find myself going back to primarily carrying around the 35 and 50mm lenses, and rarely using the 85mm. When I do use it in situations that are appropriate, I love the images I make, it just turns out to be a funny focal length for me on that body. A friend of mine has found something similar, (though with the 105mm), and has gone back to pretty much just the 35 and 50.
I don't know how moving to FX plays a role, here, as any FX camera you move to should work fine with the 50mm/1.8G (unless I'm missing something).
Manual focusing is a really useful technique to learn on cameras that give you the relevant tools to do so, but I'm not sure the D3200 is such a camera for the ways most people use it. My old film cameras had the prism thing on the view screen that helped you line things up, my NEX-5N has focus peaking and easy zooming on the viewscreen that also help. Manual focusing on my D40, though, was an exercise in frustration, as all you had to go on was a little green dot in the viewfinder. Not easy to manually focus with that when you're working with a shallow depth of field and in typical situations with people moving around. The D3200 is probably better if you use live view and zoom, but still, autofocus is really helpful. If you have the means to spend $100-200 to get that feature back, I think you'll feel like it was money well spent.
If you've still got your D50 around, I wouldn't sell the 50/1.8D, as you won't get that much for it and it can be a bit of a bother to do so. I'd just park it on the D50 and use it as an awesome spare body.