Being that gen 3 is just around the corner I'd wait before you tm (I mean you've managed to make it this far without the gyardos right?)
Edit: I saw in another post and didn't really consider about the possibility of them legacy moving hydro pump. That might change your calculus (I don't think they will but stranger things have happened.) I definitely wouldn't worry about tm on the fast move though.
As far as the raikou I'd go with the one with the higher attack because the defense and stamina aren't drastically lower and also not as important since you can't place it in a gym anyways.