What is the difference? Which is better to do?

i didnt think they they made a catback for the scion

Magnaflow makes a catback for the xB. Unless you are going forced induction I would run an axle back and save your money. The midpipe in on the exhaust on the xA and xB it big enough. Anyway the difference on the two different exhaust is an axle back is pretty much changing out the muffler. A cat back is replacing after the cat. Like I said earlier a catback for an NA xA/xB is a waste of money.

Isn't the GReddy SP2 catback?

this is a touchy subject as i have found. If you find a cat back, call the dealer if its not a magna flow or borla, they are the only for sure manufactures that make one.