I've no experience w/ the Nikons, but I've got a 10D with a 20mm f/2.8 and the 28-135mm f/3.5-5.4 Image Stabilized lenses and the 550EX flash. I've been very happy so far. No complaints about the camera except for the awkward image zoom during playback mode. Build quality is superior to anything I've ever owned, and the features I use most are the ones lacking on the digital rebel (custom AF point button most notably).

I really enjoyed using a real flash - no more 'rim shadows' around my subjects. The E-TTL system is a bit quirky and does take some experimentation before getting comfortable with it.

Stepping up from the G3 I'm sure you're going to be very happy regardless of what you choose. I chose Canon for the wider lens selection and overall lower cost - I started with the Canon A2E film back and loved it - a good middle of the road film back with some cool 'gee-whiz' eye controlled focusing. Unfortunately, scanning film and correcting for dust & scratches got old in a hurry.

My first digital was the S35 (3.2MP) which I still enjoy shooting.

-Zeke