I got the 250GB PS3 slim and have barely put much on it yet. I have the 120GB 360 Elite and have filled it up twice and had to clean it out. A lot of it is from downloadable games, but the majority is due to installing the disc games.
I'll admit that I like the PS3 and the exclusive games for it are amazing, but more people I know have 360, so I don't do much online gaming with it. That and some of the multiplayer limitations compared to the 360 really annoy me, though to many they won't matter.
At the moment, the PS3 gets used more for DVD and BR playback than anything else, though when Uncharted 3 comes out this year, I'll likely play nothing but that for a good while
For this generation, I have mostly Wii games (something like 80+, I'd imagine), then 360 games, and then PS3 - but that's bound to happen since I really only get exclusives for it. Out of all of them, the 360 gets the most play, though I've been doing more and more PC gaming as of late since I finally have a pretty decent system.