The funny thing is, 98% of people on this site have not driven even one of the cars let alone the 2.
So here comes the know it all Nickster...
QPorte.
This is really an audacious and individual choice. The plusses are great handling for such a big car and a great engine, it sounds 80% a Ferrari 430 (it's essentially the same engine but tuned for torque instead of horse power.) This car needs a Tubi though for the engine is barely audible inside. The interior is very, very exotic and individual with absolutely TONS of personalisation options, there are like 8 colours for the steering wheel alone and you can hava a custom colour made.
Negatives.
TINY trunk. It is unbelievable just how small it is in such a large car. It is tiny even for a compact car. The rear seats don't fold and there is no ski pass.
Reliability, it's a bit approximative. Nothing serious, but in one Qporte that I drove quiet a lot, the foot well leaked in rain (on your pants and shoes.) the gear stick lever came off if you pulled on it (the automatic version.) and there were annoying warning lights that came on constantly.
The automatic version blows... HARD. The suspension has been softened and even in Sport mode the car feels like a barge. The steering is extremely sharp and quick, so with soft suspension it is very diconcerting to drive. The autobox is very lame, it overrides your commands in Manual mode, upshifting AND downshifting. Say you are cruising in 3rd in manual and press a little too hard on the gas... the car kicks down to second and jerks around. The F1 box is much more sporty but a bit jerky.
The ///M5.
I only drove this car twice and to be honest wasn't impressed, but there is a good reason for that... I couldn't set up the damned car!!!! It has so many buttons and settings, that after like 20 minutes of tinkering with it, I gave up and drove as is. I think I got it into full power mode and Sport suspension settings... but the steering stayed light like a Buick. Me and my dealer couldn't figure out how to change it and it sucked big time. Since I find my RS4 slow and boring, let's say that I wasn't THAT impressed by the power, although I am 80% sure I was in Full Power mode. The SMG shifts were very jerky and far from the efficiency of the E46 M3 or a Ferrari F430. The car also constantly spinned it's wheels and just seemed loose and nervous.
However the 5er has a big trunk and decent space. It will also hold it's value better then the Maserati and will be WAY easier to sell or exchange later on for another car. It is cheaper to begin with AND more practical.
So in conclusion, the Maserat is only really for eccentrics, rich Italians or wannabe Italians or die hard German car haters and the only model worth getting is the Sport GT version.
The ///M5 is really the obvious choice here. And sounds even better then the QPorte when fitted an aftermarket exhaust.