I use Boom & Rapid Towers, what would be a better investment? Faster Towers or Extra Damage?

you will see a bigger difference with the "extra damage" with the boom tower and "faster towers" with the rapid tower as each of them have a much larger number in those areas. the reality of the math and the dps (damage per second) is going to be very very small and should just be decided by personal preference or play style.

I think you schould level both. e.g. a full ubgrded boomtower next to a full ubgraded surge tower does with 5 "extra dmg" points 1008 dmg. In most tournaments is that very helpfull in wavs with nearly 1000 hp.

@ Njulian:
well, I will level both, but which one should I level first? And what's a surge tower?

Level Faster Towers first if you use Rapids more frequently than Booms.
Level Extra Damage first if you use Booms more frequently than Rapids.

@Eklipse: Teh surge tower is the damageboost tower.