Exhausts like Apexi WS2 (I own this one) ExMag 2, and Spoon version have more midrange torque due to the backpressure and I would recommend for stock to mild build motors. The canisters are originally made for prepped engines and turbos/supercharged engines. Free flow sacrifices low end and midrange torque but if all you are going to do is race it, then it does add a few ponies in the high end. Alot of people buy the canister cat backs due to it being loud but no one thinks about the tuning of it or the negative affects having no backpressure. No backpressure also = no gas mileage. Canisters are loud and the chambers and tuned exhausts are quieter and normally 93 decible complaint, which Mr. Officer tends to like alot more.
Good luck with the purchase.
EDIT: I wouldn't recommend axle back... sorry, left that out