I'm going to predict most pollsters will pick Canon.
Either is a good choice. You just have to know what it is that's important to you, what each system offers, which feels most intuitive in your hands, and then be comfortable with your choice.
Canon is by far more popular. You'd enjoy a better choice of less expensive lenses, better high ISO pictures, and an overall sense of being part of a worldwide Canon community.
Olympus is also a good choice, and I can speak more about
it since I'm a user. In body IS, excellent quality designed for digital glass (which tends to be expensive), and lighter overall, being part of the 4/3 system (which will also make it not be as good at higher ISOs, which I take it you know since you mentioned nighttime). It's a popular brand for travelers and nature photographers. See here:
http://www.nwpphotoforum.com/ubbthreads/information/php/2007_Reviews/Isaac/Oly510Review.php The user community is not as large, by any means, as either Canon or Nikon, so sometimes you feel as if you're working for Maytag.
If that would bother you, you're better off with something else. Check out the micro 4/3 systems also, which will work with an adaptor using 4/3 lenses.
If you have doubts about Olympus' capabilities, take a look around at some of the various websites for shots taken with them, like flickr, the dpreview Olympus SLR User Forum, and the Olympus website. You'll see there's some very nice work out there. Realistically, it might take you a while to get them looking that way yourself, but that will be pretty much true of any system you buy.
Good luck with your choice.