Three major items (well, major to me):
1) HF R400 has no "lens filter diameter". This tell me two things... 1) the lens diameter will be small - less than 40mm diameter - but this is a guess since there is no specification listed - and 2) can use lens filters - not a bit deal, but also cannot use tele or wide angle screw mount lenses.
2) Manual audio gain control... the HF R400 has "normal" and "attenuator". M500 has ~12 steps for more granular control.
3) Imaging chip size. R400 is 1/4.85" which smaller than the M500's 1/3" imaging chip. The combination of the larger lens diameter and larger imaging chip tell me the M500's low light behavior will be better than the R400.
The rest of the stuff is pretty similar.
Hopefully your computer can deal with the highly compressed AVCHD video these capture.
I can't seem to get any straight answers from Canon sales about the exact advantages to the R400 over the M500, besides zoom. Is there anyone who can explain the pros/cons to each model? I'm pretty much narrowed down to one of these two.