what's best for grilling?

Braising steak or sirloin?

Sirloin would be better as braising steak is better for stewing or being cooked in liquid You could experiment with it but you would have to baste the meat alot.

Braising steak would be terrible grilled, the idea is that Braising steak is the tougher cuts of meat so the moisture it absorbs during cooking makes it tender, I doubt you'd be able to bite through it if you grilled it. Go for the sirloin.