Which factors best describe buying exhaust for E39 M5?

For me it was about 3 things.

1. Primarily Sound/Performance.. The only exhaust i read about that had dyno'd evidence of improvement in performance was the Tubi.
2. Looks. How the tailpipes looked from behind.
3. Quality. The build quality, the fit quality, longevity.. etc

And price was an issue but they all seemed to work out more or less the same money wherever you bought them from and however you built them up. Beastpower were the best price for a cat-back system that i could find.

And I am VERY happy with the Tubi Rumore.

Sound and looks, I like the fatter tips on the Kelleners and supersprint, speaking of which, I want my kelleners to get here , Josh

All of the above!

I'm delighted with my Millteks. They are not the loudest so if that's your thing you're better off with Kelleners etc. but they are gorgeous

Two words -

Tubi Rumore

its really ashame that more peole do not buy the B&B's as they are not only the lightest setup out there, but also they create the most torque out of any setup that is offered. Bav Auto has a dyno sheet from a run and it shows an increase in torque of about +10 across the band and about +6 in power.