Well, the Canon man in me says get the 500D automatically; but in reality there isn't that much to separate them...
Without knowing what you want (or expect) to shoot, I can only muse on the technical and system differences.
In short, Canon used to hold a pretty big lead on Nikon almost across the board, but in the past couple of years Nikon has really pulled it back, and in a couple of areas (more prominently in other areas of the product range) even passed Canon.
Anyway, at this level and looking at these two bodies, as above there's not much to separate them on.
The 500D does take some beating though, when you consider that it's not far off being a 50D - which OK wasn't the upgrade over the 40D it could've been, but it's still a very impressive camera, and a consumer level camera being so closely related to such an important enthusiast level body is extremely high praise.
Comparing the bodies at sensor level in terms of Image Quality is tricky - pixel density on the Nikon is lower (meaning better theoretical signal to noise ratios are acheivable), but the additional resolution in the Canon body means that at image level (ie not pixel-peeping 100% crops) there really will be no discernable difference to 99% of viewers. I dare say it'd be much the same in terms of Dynamic Range - the lower pixel density of the Nikon would ordinarilly deliver a slightly (and I'm talking a fraction of a stop) better dynamic range (the width of the range of shades which can be captured from lightest to darkest in a single image), but the Canon model scores points back with its 14bit processing. Honestly, in terms of the body's capability for decent image quality - they're probably about as close as it gets to a match. Autofocus flexibility, speed and burst shooting rate are all massively important things to consider when buying a body too (again with your intended style of shooting in mind), but it's academic in this case because there really is only a hair's breadth between them. Anyway, all these IQ and performance similarities of course rest on the assumption of using similar quality glass, and that's where it gets interesting...
...The importance of good glass can't be stressed enough. Of course everyone starts with more realistically priced optics at first, and if the bug takes hold, empties their bank account at a later date on the hardcore uncut stuff straight from Bolivia (I'm currently amassing a nice collection of Canon L series glass). This is where it's easier (once your subject matter is taken into account), to highlight differences between Canon & Nikon systems. Overall I think it's fair to say that there's a better range of balanced-price glass available for Canon bodies in the majority of situations - but of course there are exceptions to that rule, so if you could advise on subject matter we could look at it a little closer.
Beyond glass then, you just get down to ergonomics, additional features, and possibly upgrade path...
Ergonomics and UI can be surprisingly important - it's amazing how many people can find one manufacturer's button/menu layout like marmite - love it or hate it. Very few seem to get on well equallly with either.
Additional features I guess includes things like video - at first I was a hater of video on DSLRs, thinking that if I wanted to shoot video, I'd buy a video camera (and only pay for it then); but then I thought about it some more, and the prospect of having the ability to shoot really good quality video at a second's notice because it's a function of a gadget I've already got in my hand is quite cool. When you think about the creative possibilities of being able to change lenses to get the depth-of-field and focal-lengths which you'd like in your videos too, it just gets more interesting. The only downer on both systems of course is the 'part time' autofocus in video modes. Basically it means you're not dealing with a point and shoot handycam, more an instrument of cinematography. If you put the effort in, the results can be breathtaking - from both systems, just in full 1080P HD on the EOS as opposed to 720P on the Noink (
)).
Upgrade path too rests on what you plan to shoot, and how far you can see into the future. Both have got pretty flexible ways of working now, with different routes through the product range suitable to various types of shooting. Once you get to 'serious' upgrading it probably won't matter too much anyway, because you're likely to find yourself upgrading your glass at that point to.
Anyway, let us know what you're planning on shooting, and I'll bore you to death some more...
Regards,
Rob.