Triathlons

Carbon is lighter and a fuckton more expensive. Aluminum is still light as pomegranate though but ALOT less expensive. The lighter your bike the longer you can go. You cant actually make your bike itself go faster w/o different gears but you can make it easier to go faster and go faster for longer.

Basically go aluminum. The tiny bit of extra weight it has is worth it for teh money you save.