Nikon D3100 (brand new) or Nikon D90 (2nd hand)?

My budget is tight on these two choices so if anyone could help me out on which is a better choice between the two?

There's no better choice. The D3100 is a nice entry level cam with a better video option. The D90 is a motorized body for the serious amateur but it's weak on video.

The D90 now question if it is in good shape.

It was the first dSLR with HD video and set the standard for just over $1, 000 dSLR's

It has a sensor that still out performs many newer entry level cameras and many pros keep it as a backup.

If you want an advanced dSLR that can use All the Nikkor AF, AF-I and AF-S lenses, the D90 is the one. The newer entry level D3100, while a good one, can only use AF-I and AF-S lenses. The "new" D90 is the new Nikon D7000, another amazing jump in dSLR technology

The D90 - absolutely no question! It has a focus motor in the body, so can take All Nikon AF lenses and autofocus (the D3100 doesn't have a focus motor, so if you want autofocus, you HAVE to buy lenses with a motor in them, which means Nikon AF-S lenses).

Nikon d3100's video is better then nikon d90.

The D3100, while a fine entry level camera, does not live up to the standards of the D90 or
D7000.

The D3100 is the first Nikon DSLR to feature full 1080p HD! The D90 is only 720p, so the
D3100 wins here. Remember though, the D90 is now about 2 years old and was the first DSLR
ever to have HD movie capability.