I see ton of brands but really can one be better then the rest? they all look the same im sure some have a diameter difference but is it better to pair with what you have as in HKS exhaust should i do HKS test pipe?

kinda lost on how one is better then other?
only think i can think of is the weld.

 

a test pipe is well... just a test pipe. as long as the pipe is straight & the flanges aren't welded on crooked you're golden. i have 2 used Perrin EVO8 testpipes for sale right now. for a 9 you just have to have a bung welded on for your rear O2 sensor. i'm selling them for $30shipped nationwide

 

You're correct in that given the same dimensions the only other thing to look for in a midpipe is quality.. wall material, thickness, and flange.