Im having to get the rear of my 01 M3 fixed from where someone hit it. Should I go to:

1. Watson's - I know does good work, been around a long time, has great reputation. Preliminary estimate. $5000

2. Dealers new Collision center. ( fixes BMW , Porsche, Audi, Rover, Buick, Lexus) Has only been open since Feb & I have heard nothing about them. Nice looking facilities. Preliminary estimate. $3300

Which place should i take it to

I'd go with the one that you know about, assuming someone else's insurnace is paying for it. Since the "new" shop is fixing BMWs and other high end cars, odds are they would also do a good job. Ask them what their warrantee is.