Neither. You probably want some sort of offense, though.
Stall is very consistent on the ladder--it almost always wins against bad players and almost always loses against people who know what they're doing. In tournament matches, I'd suggest staying away from it due to the way the current metagame is panning out.
Heavy Offense (as in, early-game set up of screens or hazards, then sweepers) is probably the least potent form of offense.
Standard offense (basically a bunch of offensive mons that break down each other's counters and try to not give any momentum away) is effective, as is bulky offense.
Balance is also playable, but you have to do it right. You can't just have 3-4 sweepers and 2-3 walls (this will lead to heavy loses in momentum every time you have to go on the defensive, which you want to avoid at all costs in a metagame as offensive as this one). If you're looking for a good example of a balanced team that performs consistently on both the ladder and the tournaments scene, take a look at
this. Kind of shameless self-promotion, but whatever, I'm doing it to make a point. Notice how I don't actually use walls, but rather pivots that can either keep momentum on my side (Regenerator Slowking helps a ton) or by still being very threatening offensively (Rhyperior and CBLax). Then I have my offensive core of SD Heracross (which is my main wall/stall-breaker) and Scarf Mienshao (to act as a cleaner and revenge killer).
I've recently been playing more offensive teams, though, and I've actually found them to be even more consistent, which is why I suggest you start out with offense.