You can't go wrong with either. Personally, I got the D3100 just as the 3200 was being announced. I'm price-sensitive and the incremental differences weren't enough to push me into the later model.
I love the D3100 - I probably pick it up more than my older but sturdier D300. It's light, small and unobtrusive. Put it together with the likes of the 35mm f1.8 DX G and you've got a 50mm equivalent set-up that evokes my days of wandering the streets with a tiny Ricoh, but with all the advantages of digital. With the Picture Controls set correctly, I shoot NEF + JPG, keeping the RAW for insurance, but mostly just taking the JPG and running just a very slight correction for WB and sharpness. Perfect and minimzes the amount of time I spend behind the PC.