Prior to my purchase of an '02 Coupe, I had three consecutive M3's; two E36's and one E46. I agree with most of what has been previously posted. The M3 is more cohesive and complete in its purpose than the Maserati. You get the impression Ferrari really wasn't sure what they wanted to do with Maserati; a Jaguar in Italian clothing or a junior Ferrari?
What was born out of that tentativeness is a hodge podge of sorts. One of the world's greatest engines, to be sure. In this department, the Maserati has it over the M3. What the M3 makes in total horsepower (333) the Maserati makes in torque. The Maserati curiously is very aggresively damped, much more so than the M3. Thus, a cobble-stoned road is felt much more in the steering and cockpit of the Maserati. Thus, I prefer the feedback provided by the Maserati. I say, "curiously," because the Maserati is relatively softly sprung, much more so than the M3. Thus, the M3 exhibits much less body roll in cornering, which among other things makes it the better handling car. The M3 is better at putting its power down effectively as well.
As you move from the more objective comparisons to more ethereal, the vote overwhelmingly swings in the Maserati's favor. First, the car is put together by people, for better and for worse. It's old fashioned in a good way, much the way a fine Swiss watch is, or tooled Florentine leather. No two cars are exactly the same. This makes the car less of an appliance and more a work of art. Flowery prose, yes, but true! There is very little plastic in the interior. Everything is stitched together, like cars used to be. Does this make for a more squeak prone interior. Yes, and a $50,000.00 Patek Phillipe makes little ticking noises.
For me, a frequent traveler to Europe (especially Italy) and a lover of anything finely crafted, the Maserati is much more fun to get into and drive everyday than my M3 ever was. I've previously posted that it is not for everybody; if you're not passionate about the car, the shortcomings may get the best of you. But if you consider yourself an enthusiast; if you roll down the windows when anything Italian is about to drive by just to hear the tenor of the exhaust, you're among the initiated. By the Maserati.
Just my two cents.