I had the same choice as you between D3100 and 500/550D - I would have gotten a 50D if I had the spare funds as I'd used a friend's and liked it.Anyway, I ended up getting the D3100 as it was £220 ($320) cheaper than a 550D due to duty free and supply, which was enough to get a nicer lens, and I
felt the image quality was better than the 500D's. For your questions:
1)I'm not interested in videos
D3100 is better for video, but since you're not interested, it's irrelevant.
2)Resolution: a megapixel more though a smaller sensor seem not to make a huge difference
1MP difference will make no measurable difference by itself at 12MP - 4% extra in each dimension to crop with isn't anything. What will is how useful each pixel is - sharpness noise etc, and I don't see much difference between them, maybe an edge to the D3100, but that's probably because it's what I have!
3)RAW 12bit (Nikon) - RAW 14bit(Canon) seem not to make a huge difference, you said both take good pictures.
No difference - the extra bit depth won't make much difference - you'll likely downsample to 8 bit anyway while you have the body, and even if you didn't 2 extra bits only means quantisation noise/detail loss is reduced if the sensor can resolve with that granularity (no idea if it can)
4)Canon sells cheaper lenses than nikkor, that could make a difference (is that true?)
From the few lenses I looked at it's true, but then the first body you get is not going to lock you in to either, unless you buy thousands' worth of lenses.
5)Canon: better display ... not bad, but i don't mind too much
This is something that CAN matter - if you're using manual focus in live view (useful if you want to use contrast detect AF and focus anywhere in frame, which you may use. More importantly, useful for low light manual focusing as the optical viewfinders on both are a bit crap in low light), the D3100's rear LCD does not have sufficient resolution to be confident in focus accuracy. The 500D's one can be used, though AFAIK it doesn't have contrast detect AF for anywhere in frame
6)Auto switch off display sensor: cool but not so important considered that i'd like to get a eyepiece magnifier (are there any of them for canon?)
There should be some for Canon.
The display on the D3100 switches off after 10sec, so you won't notice it anyway.
7)Bracketing: I do mind it, is it possible to get good results doing it manually with d3100?
Yes it is possible, and it is easy - after all it's just a shutter/aperture/ISO adjustment away. Convenient is another matter, because you obviously have to choose which to adjust and then actually do it yourself as opposed to have the camera do it for you.
8)both fit well in my hands
This is what I would have said you should make the choice on, but same for me, I liked both too.
9)I could get the canon for 610 (790$) and the nikon for 560 (720$): a small difference but i don't want to spend 50 more for nothing.
TL;DR you will be happy with both! I chose D3100 because it meant I could get an F2.8 lens to replace the kit one with straightaway, but had I not had that I would have tossed a coin
