how much pitch/roll rate is best for a beginner is very debatable: too quick=overcorrections, too slow=overcorrections. imo, regardless whether you use flat or symmetrical, a beginner can adjust pitch enough to get the pitch/roll rates where he/she likes them.

i've trained a few pilots who did better with symmetricals during the hovering stage. i've also trained a couple other pilots who prefered the slow response of the flat bottom blades. i prefer letting beginners figure out what they are most comfortable with, and try not to recommend which type of blade is best. instead, i try teaching them what vortex05 said: how each type will affect the heli's performance.

one more thing:
keep in mind that if you have a heli setup for one type of blade, you'll have to change pinion gears when you switch to another. ie: a cp pro is setup for symmetricals, so you have to drop a tooth on the pinion if you bolt flat bottoms on it. on the other hand if you have a cp, which is setup for flat bottoms, you'll need to add a tooth to the pinion when you bolt on symmetricals. the reason for all this madness is flat bottoms create the same lift at lower speeds, so you need to drop a tooth with them to keep the motor in it's "power band".