1) According to DxO labs tests, on the D90 (they have no data regarding the D3100) the AF-S f/1.8 G 35mm reaches 42 lp/mm and 50mm 1.8G only 39; but the 50mm has slightly less distortion and chromatic aberration. On real use these lenses have similar sharpness (they are both *excellent*) when used on DX bodies like your D3100 (on FX bodies the 50mm has a clear advantage).

2) Longer lenses have better out of focus background. The bokeh *quality* of them is similar, but the 50's is a bit smoother due to the longer focal length.

3) There isn't an "advantage", they are actually quite different. One is longer, the other is wider. If you intend to have an "all rounder" prime DX lens, than the 35mm is more appropriate. If you intend to do mostly portraits, the longer 50mm has more out of focus background.

4) The real advantage of these primes over zoom lenses is the large 1.8 aperture, excellent for low light necessities. But if all you care is bokeh, the AF-S 55-200 VR actually has better bokeh than *both* - for less money. Both attached pictures were taken with a D5000 and a 55-200vr lens at 200mm @ f/5.6, 1/200 shutter speed, ISO 200.