This cam has 2 shooting mode options as mentioned above.
Will MP4 at 35Mbps have better quality than AVCHD at 28Mbps?
In theory it should, and one view here claims it's marginally better: http://forum.videohelp.com/threads/360883-Canon-HF-G30-which-of-AVCHD-and-MP4-gives-best-quality
Here's a comparison video on YouTube that you should watch at full size 1080p:
Congratulations, nice camera. AVCHD is a special implementation of MPEG-4 H.264 compression and it's limited to a max of 28 Mbit/s. Since the G30 MP4 and AVCHD both use H264 compression, at the same frame rate 35Mbps is theoretically better, like Nick says. It's more complicated than that. Note the (PAL) G30 24 Mbs MP4 is 25p, which possibly could give you better quality per frame than 50p 35MBps.The AVCHD 24MBps is 50i. You probably also know if you shoot at 24Mbps you can back it up on the second card at that rate. Of course, you can record LPCM with AVCHD, if that's important. I think ultimately what you should use depends on the final destination of the video, if you're not sure, 50fps MP4 would give you the best compromise with distribution options and quality, but of course there's no dual recording at that rate. Whatever, there doesn't seem to me to be much point in shooting AVCHD at 50 fps.