That Porsche gear looks like it might be a hypoid gear (the two gear axis don't intersect). Given the same width, the helical gears will be stronger, the running tooth width is wider giving it more strength. Helical gear teeth 'roll' or ease into mesh and out of mesh, this makes them quieter and smoother. Straight cut teeth go into and out of mesh all at once. This creates a bit more noise and vibration because the contact of the tooth width happens all at once. Efficiency, I don't know. I've seen Kyosho, Losi, and Serpent cars free spin very well and they have straight gears. I've seen my HB car spin very good with helical gears, but not as good as the Serpent I saw. Efficiency under load when the car is on the track could be a different ballgame but I think the gear cut style for r/c cars for the end user realistically only comes down to durability (how often you replace gears or gearboxes to keep the gears from skipping).