The only downside to the bigger cameras is, unless you spend the big bucks, you're getting smaller apertures. My Canon G3 lens opens all the way to f/2.0 (at the wide-angle end). My much pricier Nikon D70 kit lens opens to f/3.5 (again, at the wide-angle end). That's a difference of 1.5 stops. On the other hand, the Canon has to run at ISO50 to get optimally low-noise photographs, where the Nikon doesn't even go lower than ISO200.

Which is to say, that it's something of a wash. You can push the Canon G3 to ISO400. You can push the Nikon D70 to ISO1600. In both cases, that's three stops. I've found the G3 to be usable at ISO200 and just awful at ISO400. Eyeballing photos of mine, I'd say the G3 at ISO200 is similar in noise to the D70 at ISO1600.

So, trying to hold things the same (smaller camera, brighter lens vs. larger camera, dimmer lens, better sensor), I'd say the bigger camera only really buys you one stop of extra light. Of course, you can spend extra bucks on a bigger, brighter lens for the bigger camera. You can also get one of these vibration reduction lenses that, in effect, buys you another two or three stops by letting you hand-hold the lens at slow exposures.