Well, the reason your square taper cranks kept getting loose was because you probably didn't tighten them enough in the first place, using a torque wrench. Having said that, IME square taper cranks when new need to be re-tightened a few times before they finally get seated, because the bolt loses some of its pretension force as the crank arms creep a small amount along the spindle. So, to that extend I suppose splined cranks are more convenient in that they don't creep and once tightened properly they stay tightened. The problem with splined cranks is that they need to be made very precisely to avoid play between spindle and crank, and cheap crap like FSA often have problems. If you buy something good like Dura Ace you won't have any problems. Myself, I'm a 100% square taper guy from way back, and have never had any problems. As to strength, well if Sugino 75 NJS square taper cranks are good enough for pro keirin, they're certainly good enough for you and me.