While there are a lot of ways to approach this question, the two things that stand out to me, in this modern and competitive market (where many of the brands are very similar in many ways, especially "quality", however one chooses to define that), are these:
- Ergonomics.
Different brands have different controls -- from the positioning and type of buttons and dials, to the "UI" (user interface) for the on-screen menu systems, to the shape, weight, and general ergonomic considerations of their products.
Camera models within brands will differ in these features, as well, though there tend to be some characteristics that tend to be more-or-less consistent within a given brand (e.g. the positioning of dials -- Canon and Nikon (and presumably the other brands, too, though I have less experience with them) place dials in different positions, which means finger positions are different for operating them.
Things such as this are very much a matter of personal preference -- depending on obvious things such as the size of your hands (more likely to influence camera models within a brand) to subtle differences in how your hand operates that are hard to characterize (more likely to influence brand choice).
To evaluate the ergonomics of the cameras, my recommendation is very strongly to try out the different brands you're considering -- hold them in your hands, shoot some photos with them, try changing various settings, etc. Only through this sort of hands-on experience can you truly know if the ergonomics will "work" for you.
Anecdote: a couple decades or so ago (well before the age of digital), I did some research when I was gearing up to buy my first SLR. I'd liked my mom's Nikon FM (an old all-mechanical classic), and figured I'd buy a Nikon as well -- Nikon seemed to be the leading brand at the time, it was what my mom had, and various things pointed me in that direction. So, I bought one (perhaps an N6006? I no longer recall), and was of course ecstatic to have my own SLR... But I found that the feel of it somehow just "wasn't right" -- I didn't find the positioning of dials, settings, etc., to be intuitive to me, and I therefore ended up regularly fumbling with the controls. One of my peers at the time had a Canon EOS-line camera, and I tried his out, and I found that it just seemed to "fit" me better. The controls made sense to me, etc. So, I returned the Nikon, and got a Canon instead. I've primarily been a Canon shooter ever since, having re-evaluated my decision when I was first entering the world of digital, by trying out both brands.
Now, lest you think this means my answer is "go with Canon", let me just say: I've known others who have had the exact opposite experience -- They've gone with Nikon specifically because the ergonomics "made sense" to them, and/or "fit" them better. So really, it's an individual choice thing, and you need to try the cameras out to figure out which one you're likely to prefer.
- As has been mentioned in other answers, the ability to borrow equipment.
I consider this to be by far a secondary factor to point 1, especially if you have a wide circle of photo-enthusiast friends (since in such circles, there are bound to be folks with various gear from any given make), but it is quite handy to be able to borrow a lens here and there, or a dedicated flash, or even a battery if yours runs dead while you're out with friends.
So, this might lean you towards Canon, since you say more of your friends have them. Still, I'd say ergonomics is a much bigger factor.
There used to be a 3rd point, too -- full frame sensors. For a while, Canon was the only one doing them or planning to. When I switched to digital, this was actually part of what kept me with Canon. I wanted to be able to use the same lenses on my Canon film body and digital body, and wanted them to behave similarly. I still have yet to actually purchase a full-frame body (though I expect my next digital will be one), but I knew I would want to one day (and I do, and I will do so, I'm sure). At the time I was considering the switch, though, Nikon didn't have a full-frame offering, and as far as anyone could predict at the time, they never intended to. Many of their lenses were different between film and digital, and this just seemed limiting to me.
Of course, times have changed. Nikon is now very much into the full-frame market, and Canon sells a number of lenses that are digital-only. Still, something to maybe be aware of... I haven't looked at Nikon's lens line-up closely enough lately to know if this would be something that would matter to me today if I were making the choice again (as for Canon, I generally don't buy EF-S (digital-only) lenses, because I want my lenses to work on full-frame and/or film bodies.
I hope this helps! (And ooh, look, as I was writing this, another vote (answer) for ergonomics came in. Ah well, still figured this was worth finishing.)
And good luck on your entry into the world of DSLRs!! May you get many years of enjoyment from it. :)
