Which kit should I go with? Nikon D90?

Should I go with the D90 with the 18-105mm lens or the D90 with the 50mm 1.8 lens?

I would go for the 18-105mm for general photography or the 50mm if you do a lot of portrait photography. I'm a big fan of prime lenses though.

I agree the 18-105 - very versatile lens. The 50 (I own Canon and have a 50) is a good lens but limited.
It's an excellent portrait lens but with no zoom at all - you walk.

I'd go with the zoom.

You would get more use out of the 18-105mm.

If you can only choose one or the other, I'd go with the 18-105mm as it is the more versatile lens. However, I strongly recommend that you also get the 50mm f1.8 (maybe even a used one) down the road once you saved up enough for one.