Nikon 3100d, Pentax k-r, or Canon 550d

I've now been using a dirt-cheap bridge camera for 2 years, started As photography at college this year, and i think its time for a big upgrade! I have narrowed down to the 3 cameras above and have a max.budget of £550, i think the best idea is to get a second hand body and get descent accessories later, i like the nikon 3100d because it seems to be a popular DSLR and specs aren't too bad, Penatx kr because it's fast, colours are good but i don't think ISO performance is great! Whereas canon 550d, isn't that fast but i think its got a much better ISO performance. Please Help! And feel free to give me more cameras to have a look at, Thank you!

You really can't go wrong with the 550D. It's a great all round camera, and for the price its probably the best camera in its price range out right now. But when investing in a DSLR, you are investing is the lenses too. A camera body is going to become out dated within a few years. But lenses are what make the image, and they don't really go out of date as such. I've owned both Nikon and Canon systems, and I'm with Canon right now, and I think I will be for a long time.

If you're looking at second hand surely you shouldn't look at the nearly new cameras which are still fetching good money. Perhaps it makes more sense to look at an older generation of cameras such as the Nikon D80 or Canon 40D or perhaps the D200. These three are semi-pro DSLRs built well with all the extra functionality that is useful for the photographer.