Looking to spend about 500$ on a 2.5" catback, which brands are the best and what makes them better than a different one? Want it sound decent

YOUR OPTIONS INCLUDE:

XFORCE
REDBACK
MANTA
CUSTOM FROM EXHAUST SHOP.
STOCK XR6 TURBO

find out the rest on Ebay and google.
People like mild steel (when it warms up) over stainless.
Get two mufflers or only one if it want to be heard clearly (and you don't mind a bit of drone).
Extractors are also a v. good addition.