I agree with most of what Scott says, but let me add possibly more than you wanted to know:
In theory, helical gears should be stronger because their contact ratio is about 1/3 higher (approx 2.1 vs 1.35). In the real world of racing however, straight-cut gears are quite a bit stronger. This is why F1 cars (in which gears are less than 1/2" in width) utilize straight-cut gears exclusively ... as does virtually every racing transmission that has seen 10+ years of development (or borrowed from past development).
As a straight-cut gear tooth engages, the load is spread across the full width of the tooth from the beginning of its engagement until the end of its engagement.
As a helical-cut gear tooth engages, the load begins at one side, and works its way to the other side of the tooth as the gear pair rotates. This side-to-side engagement sets up a tearing motion (as clearly evidenced by the way in which a helical gear tooth breaks).
This tearing motion is compounded by the fact that all idler gears require clearance to rotate on their respective needle bearings. The greater the clearance, the more the gear is cocked when heavily loaded, and the further away the gear pair is from ideal mesh. (The real bitch is that racing transmissions, which run at ridiculously high temperatures, require a tad of extra clearance on the idler bore to avoid seizing.)
A straight-cut gear doesn't care how much clearance there is in the idler. Because the load is always distributed across the full width of the tooth, it won't cock.
Straight-cut gears are usually also dogged gears (large engagement lugs, rather than small synchro teeth) ... but it is a misconception to believe that these two features must go hand-in-hand. There are synchronized straight-cut gears, and there are dogged gears (both H-pattern and sequential) that have helical-cut gear teeth.
Off the soapbox now.
