Hi, Donna
I bought the D5100 from Costco about a month ago. My daughter-in-law bought the D3100 at the same time. We both bought them in kits with two zoom lenses. Mine are 18-55 mm and 55-300mm. Her short lens is the same but her telephoto is only up to 200 mm. Her kit was about $750 and mine $1,000.
I found this website great for comparing options:
http://snapsort.com/compare/Canon-T3i-vs-Canon_EOS_Rebel_T2i You can put in any cameras you want. It rated the D5100 at 95 and the D3100 and Canon T3i at around 70. But you may not agree with the criteria. I do not see that much difference between the Nikon D3100 and D5100.
We are both delighted with our purchases. I start a class next week so have been diving into the instructions and playing with it...a lot.
The main differences I see, other than the lens options in the kits, are that the D5100 has a rotatable screen and the D3100 has a "Guide" mode that gives helpful instructions. There are other minor technical differences; e.g. sensor size and ISO range that I don't think matter much. She has less experience than I and thus really appreciates the Guide mode. I would have found it wastefull afer initial orientation. I am pretty sure most people would be equally happy with either camera...or the equivalent Canons.
I have been taking pictures a long time with many different cameras. I still have my old 35mm which I now realize was a simple manual-only camera. All it has electronic is a built in light meter that you had to set things to get a needle horizontal...usually by twisting the f-stop ring. The things you can do with these new cameras are amazing.
Its going to take me several years to learn how to exploit the incredible options on this machine. I have already been taking far better pictures than I have for the last forty years. I am really excited about it.
I also have a relatively new "point and shoot"...Panasonic Lumix DMC-ZS7. Interestingly it has many of the same features so I am learning more about it as I learn how to operate the D5100. It of course is much smaller and lighter and thus more suitable for some situations. It has a gps which is really cool...but there are $50 non-Nikon GPS options to plug a gps into the D5100/3100. I have one on my list.
So I already have this list of options to add on to the D5100, including the above. The first one I will get is a flash. (Well, second...I have a remote release on its way...$13). I have already found that there are a mind-boggeling array of options for just the flash. Its like buying a flat screen TV. ;-( I don't plan to spend the money Nikon wants for theirs. I'd appreciate any input on that.
Next will be a polarizing filter.
Anyhow, hope these thoughts may help in your decision some small way.
Regards,
Larry Leach