I think the tri-flow has the best sound and is of very high quality.
You can get an exhaust that may give a little more of a power boost, but for normally aspirated street cars like a 325i, an exhaust is more of a cosmetic addition than a performance one.
I bought the tri-flow for the sound and the appearance. If I had money to burn, I would buy one and stick it on the wall of my garage or basement as a piece of art.
$643 is a very good price for that exhaust. Mine is rock solid, with stainless steel welds in addition to construction, unlike some others.