Helical gears are stronger most of the time IMO. They have more contact area, and its normally cheaper to replace pinion bearings than the pinion gear.

For instance, I just rebuilt the diff's in my brushless MGT. It has helical cut diff gears and they look great still, but the bearings were getting worn after a lot of use. $10 later and they are good as new. If the gears were shot, it would have cost me a lot more...