I'm not sure what you mean about higher head speeds. On a 450, you shouldn't need to go over 3000rpm on the head speed.
The straight cut gears are stronger, but the helical cut gears are quieter. To put it into perspective, the transmission in your car uses helical gears, but your engine is only pushing about 200hp (guessing). This provides a very quiet ride, you don't even hear the gears. However, in a race car such as NASCAR, they use straight cut gears and are pushing over 600hp. The transmissions in those are very loud and you can clearly hear the gear wine from inside the car.
IMO, unless you are just smacking the crap otu of these with the most powerful motor you can find, go with the helical gears. I would suggest, with a 3500kv motor, that you run the 11T pinion. The tooth count on the main gear may be lower, but the teeth are thicker resulting in a smaller pitch number. This is the way they make them strong enough to withstand the same amount of power. Hope this all makes sense.
