Which do you prefeR? Nikon or canon?

Nikon d3100 or canon 550d?

I don't recommend one brand or the other. Both Nikon and Canon make good cameras, it has more to do with the actual photographers skill/knowledge.

The best thing for you to do is to go to an actual camera shop (not an electronics store). You need to hold/handle/try the different brands and model in your budget to see what feels comfortable to you, and that you can find/use the controls easily. I chose Canon for its ergonomics and menu layout while a fellow photog chose Nikon for the same reason. Don't forget to budget in memory cards, extra battery, a padded camera bag, and taxes into your budget.

If you go to http://www.yellowpages.com type in camera and your city, state you can see if there are any camera shops near you. I personally avoid Ritz/Wolf though.

Canon

There's no real quantitative way to describe any differences between the two camera's processors, largely because they're highly proprietary and very different. It might be worth noting that the EXPEED 2 is brand new while the DIGIC-4 has been around since 2008, so perhaps there's two years' more technological advancement in the D3100. That said, the biggest change since the EXPEED is most like the addition of Full HD video processing. At the end of the day, both processors power much more hardcore DSLR models, so they should both easily suffice at this level anyway.