Would you rather have a player with poor speed and great baserunning, or a player with great speed and poor baserunning?

Easy, the guy with good baserunning. If a guy has good baserunning and poor speed, he won't steal much, but when he does he probably won't get caught often. But the real benefit is that that guy is going to hit a lot more doubles and triples, and make far fewer baserunning mistakes.

The guy with the speed will leg out more infield hits but have a SB percentage that makes him a liability.

But you can control how aggressive your team is on the basepaths, so if you're going to be careful stealing and taking extra bases, then speed may be the better ability.