I'd make sure you do a few test drives before you make the jump. I made the jump from a 135i as well and to be honest the M3 "felt" slower than my Dinan-tuned 135i. In reality I they were really close in straight-line performance. Torque is deceptive. If you just want the car to go/feel faster, I'd just go with a tune. Go with Dinan if you are worried about warranty issues. I'm pretty positive you'll be extremely happy with the power.
(1:40 @ Laguna Seca, 2:09 @ Thunderhill, 1:56 @ Infineon)