Straight-cut gears aren't really stronger, but that's close enough for most people to understand, I'd say. The downside of helical gears is they
transfer some of the load axially, which transfers stress to other components down the line (bearings, for example) that will then fail faster than they would otherwise. Helical gears are easier to shift and, of course, much quieter, so at the loads that a street-driven car produces the additional axial loading isn't too hard to engineer around.
Automotive transmissions are constant-mesh. The gears don't "shift" at all. Instead the shifter moves the locking collars that lock various gears to the shafts - but all forward gears are meshed all the time. (Yes, this means that when you're cruising down the highway in 6th, 1st gear is happily spinning along at X0,000RPM.
