Which is better a nikon D5100 SLR camera or a nikon D3100 SLR camera?

I really want a nikon SLR camera but i don't know which one is the best

The D3100 is probably your best choice, judging by the nature of your question

If you can't tell the difference, it might be better to save your money and go with the D3100.

The better of the two is the D5100-by a lot of quality. What they are telling you is that if you can't understand enough to tell the difference then it won't really matter to you.
This very very basic comparison should be able to tell you the basic difference
http://snapsort.com/...ikon_D3100

Spec-wise, the D5100 is better. Price-wise the D3100 is better. The best Nikon dSLR camera right now is the D3x. It is the best because it has the largest sensor size among Nikon dSLRs. Work with what you know and what you have. They are all basically the same anyway so it wouldn't matter much whatever level of proficiency you have.