If I could have justified the price difference I certainly would have gone with the /M, but I couldn't. $20k difference on a CPO model of the same year just wasn't worth it. 50i + dinan tune isn't far behind a stock /M, and has a better torque curve to boot if I am not mistaken.
Plus I love the 8 speed ZF.
Edit: it helps I have an e92 M and a money pit S4 to play with when I need to go really fast. The 50i was the best of both worlds for my budget.