Be sure to get the 1.8G and not the D. D3100 wont focus the 1.8D
Check your craigslist, should pull up some decent used ones. I got my 35mm 1.8G for 150 pretty much brand new
If you shoot indoors a lot I would recommend the 35 as the 50 is a bit long (remember the crop on the D3100 is 1.5x) Both are great lenses.
Honestly, you should walk around and shoot with your kit lens at 50MM and not touch it. If you like it get the 50MM, if you don;t have enough reach for your sports or animal photos, get the Nikkor 55-200 VR lens for some reach.
That is what I have been doing with my kit lens. And as for the shooting I do a lot of outside, more than inside so I think I will go with the 50.