I actually cobbed my xi last night just to see the difference and to get a longer impression of higher torque than the is test drive allowed. I like. But I also got complements on my Monaco blue / saddle brown xi coupe at the gas station which leaves me with a "can't go wrong" feeling.
I am leaning towards the is purely for the deal and higher torque around town. If I crave the m3 color combo I still have time to get a European delivery in August / september and sell the IS at not too great a loss. A bit silly, yes, but not too financially stupid. I think.