35mm vs 50mm with Nikon D90

As posted in a previous thread asking about infant photography, I'm getting close to finalizing my decision on a lens or two to add to my collection. Standard set-up on my camera is 18-105mm f/3.5-5 with SB-700 AF Speedlight Flash.

I've read probably a hundred pages on the inevitable 35mm versus 50mm question. Though the prices aren't prohibitive for buying both, I know myself and know I won't be changing lens frequently with a child on the way in a month. Primary use with obviously be taking pictures of my newborn daughter and family/friends holding my daughter.

I've listed my options below. One important note: the camera, speedlight, and lens will probably be heavily used by my wife and in-laws as well. So the priority is going to be how well everything plays nicely with auto-focus in both speed and quality.

- Nikkor 35mm f/1.8G AF-S DX    ($197)
- Nikkor 50mm f/1.8D AF Nikkor  ($125)
- Nikkor 50mm f/1.8G AF-S FX     ($219)

Which lens would be the best if permanently attached to the camera for both portraits and candids? Especially on auto-focus. If it's strongly suggested I buy both a 35mm and 50mm (as many sites say), what's the compelling reason and how much (which shots) should I be switching back and forth?

Thanks all for the opinions. Don't have much indoor photography experience and just want to make sure these shots come out well, especially since I won't have control of the camera for many of them.