Quality

The craftsmanship of an exhaust can affect its longevity and durability.  While I live in California and rarely if ever encounter road salt, I still was interested in only the highest quality exhaust.  Specifically, the exhaust had to be of exceptional quality with strong welds and quality construction.

Style

An exhaust canister (muffler) should never look "riced out" so that the appearance of the exhaust encourages street racers to rev their cars at you (the look of the STI does this sufficiently on its own).  I preferred a straight line tip exhaust over an angled one only because it looks OEM and in California (and possibly other states) a modified exhaust with a straight tip is less likely to attract a police officer's attention than a modified exhaust with an angled tip.

Performance

3 inches, that seems to be the agreed upon standard diameter for an exhaust to perform at optimal levels in most turbo applications.  Even though the 3 inch diameter would not likely result in a great increase in performance on a catback system, it does support the future addition of a 3 inch diameter downpipe for maximum performance.