Any of those are good starter cameras and you will get a lot of use out of them. I've personally have never shot with a Nikon, but I know plenty of people swear by them. I've owned my Canon T1i for several years and I still feel that I haven't been able to get the fullest potential out of it. Plan to invest some additional money in lenses after a while too.
This is a great site to read up on digital photography reviews:
Digital Photography Review