The speed3 is lighter and have less parts to worry about breaking. The speed6 on the other hand has far better traction and you won't have to worry about wheel hop anytime soon. You just have like 300 more lbs to lug around. You can do about the same thing to both so really, it's all on what you prefer. I'd probably go for the regular mazda3 and boost it only to put down even more power than the speed3 with less boost.