450D is actually a bit cheaper about 400 with a 14-42 mm lens while D3100 is 580 with a 15-55mm lens and 500D with 15-55 is 650...
but i just noticed that there's is another model in the middle: canon 1000D 500 with 15-55... but it's 10 mp.
So I update my question: What should i choose between Nikon D3100 and Canon 1000D or eventually Canon 500D?
Does the Canon 550D really worth the 200 more than the D3100?
thanks, i'm being so annoying with my questions and my bad english :mrgreen:
mmm... some confusion here

.
Canon and Nikon kit lenses are 18-55mm; 14-42mm is typical of 4/3 (Olympus). Kit lenses for Canon and Nikon come in non stabilized and stabilized versions (IS for Canon, VR for Nikon); stabilized are better, at least in the case of Canon also optically.
In Canon line, 1000D<450D<500D<550D (although 1000D is still selling, middle numbers are out). By the way, 1000D might be found for less than 400; 550D now online goes for about 700 (and 500D is about 560).
While one year ago the basic choice was D3000 vs. 1000D and the latter was better in reviews, D3100 is a really good step from Nikon (I often read these reviews:
Nikon D3100 Review: 1. Introduction: Digital Photography Review ). Compared to 550D, the lower price make it better for me. However, I would also look to D5000, if you are not interested in full HD.
Regarding d3100 vs 550D, here there is a summary of differences:
Nikon D3100 vs Canon EOS 550D (Rebel Ti2) | Digtal Camera Reviews - The Imagery Resource! (add also that d3100 has no bracketing, so if you are interested in HDR, you have to do it manually; bracketing is available in all Canons and d5000 too).
In summary, any of these is good for learning. The least expensive let you save money for other extras (a second lens, a flash). The most expensive are also good for full HD video (with slight edge for 550D, which has external mic input).