The most important factor is progressive overload. Increasing the intensity/weight/reps/etc consistently. You can gains mass/strength with all kinds of rep ranges. Some people respond and/or prefer some over the others. I personally mix it up. Try them both and see what you like best. As far as the science goes, in terms of pure hypertrophy, 8-12 is supposed to be the optimal rep range give or take. I have found success at pretty much all rep ranges. They can all work, just find out which one you like best.