Aiming is obviously more important. Reason: aiming is how you hit with accuracy, and when you hit enough you kill, this gets you points and your team points, thus winning the game.
However, most people can "sort of" aim and hit a tank. At least a little bit...
But I have seem MANY MANY tanks that cant drive and evade shots from people. This is more defensive play and it is ESSENTIAL to becoming a good player. If you can make the highest skilled players miss BECAUSE of your movement then you stay alive longer and have more chances to kill them. Thus, these two are inseparable...
I think dodging is harder to do, but aiming while moving and remaining accurate is VERY tough... and here i go again into the chasm of reciprocal statements...
what came first the chicken or the egg?
