Pull apart any aftermarket exhaust, any it's clear that there's not a lot of high-tech engineering going on. Stock systems get complicated because they have to make acceptable power AND legal decibel levels, not easy!
What you should be looking for is visible quality in the fit, finish and materials. Look at as many pipes as you can yourself, check the welds and ask how well things fit together. Generally, I'd stay away from carbon sleeves, they're too brittle. Aluminum is great, SS is heavy and holds heat to well, Ti is nice if you want to spend the extra $$.