Which is a better buy for your money?

Nikon D3100 for $700
or
Nikon D5000 for for $650? In reviews they always say that the D5000 is a step up from the D3100.

I'd say the Pentax K-x is the better buy "out" of the two.

The reviews should say that the D5000 is a step up from the D3000. The D3100 is better than either the D3000 or D5000 feature wise and has Full HD video.

The most bang for the buck is the D3100

Have you looked on the Nikon website and looked at the features on each?

Doing the research on your own will assure that your understanding of each of the cameras features so you don't get confused by "they" in reviews written by people who are NOT photographers and do not know the difference between motion picture and video

Fhotoace has it right!