Ya know, I'm not a super camera guru. I just did a LOT of research on DSLRs for beginners and by and large the consensus at the time was the D3100. I loved it and went to get a 4th or 5th lens for it and ended up messing with the D7k which just put it to shame. So, I got it.
I couldn't tell ya why so I guess I'm not a lot of help but, after lots of research between the rebel and the 3100, I decided the Nikon was the better fit for me. Then just moved up. Lens selection may have had something to do with it too, I don't quite recall.
But I really don't think you can go wrong with either - they are both highly approved cameras and as a beginner we may never know or care the real differences.
Accessories can be found for either.
Rob has the Rebel btw, so we have been known to bust on each other's pictures. :P