From a perfomance stand point, the M is a no brainer. Maintenance wise, the 330 is it. If the M was still in warranty, I'd say go for it without thinking. But being a post M owner, Leave em when the warranty expires. I had no major problems from mine. In fact, my neighbor behind me now owns it and still raves about all of the Dinan upgrades. But my issue would be that you don't know how the last owner treated it.
Ultimately the decision is yours. Good Luck and keep us posted..................Phil

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2011.5 Dinan ///M3:
SSII DCT
Competition Pkg
Convenience Pkg
Premium Pkg 2
Heated Seats
Enhanced Premium Sound
Black Kidney Grills
BMW M Performance Steering Wheel
BMW Carbon Fiber Performance Spoiler
BMW Aluminum Pedals
Dinan Exhaust
Dinan Mid-Exhaust (x-pipe)
Dinan Carbon Fiber Intake
Dinan Performance Tune
Dinan Pulley Kit
Dinan CH-R Wheels
Dinan Carbon Fiber Strut Tower Braces
Arkym Front Lip
Carbon Reflectors
Lux H8 V3
Brembo 365mm BBK