Go for the 30D, the extra mega pixels don't make up for the build quality/extra performance you are losing by having the 400 (unless you have very small hands!!). Just something to consider, when shooting anything with a blade (helo or aircraft) bring the shutter speed down from what you would normally. The first shot has frozen the rotors too much where as the inverted shot in the second looks a lot more natural.
However if you are shooting someone doing extreme 3D like Alan Szabo Jr (probably my fave pilot) or Curtis Youngblood then you will never get anything captured with a slow shutter. For those of you who have not seen what the top RC heli guys can do just look at my hero Szabo Jr doing his stuff!!! http://www.youtube.com/watch?v=3iWIR...ted&search
= and yes this is all real!!