I had an mc shift Granturismo S before my Scaglietti. I'd have said there may be ever so slightly more legroom in the Maserati but quite a bit more headroom in the rear seats of the 612. Either way you can certainly take a 5ft 10 inch driver and same size passenger sat behind fairly comfortably in the rear, perhaps the steering wheel had been extended in the one you sat in forcing the drivers seat back. I have two child seats in the back of mine, the children actually take up more room than an adult sat on these as their legs stick straight out and we've had no problems at all with the car acting as a family saloon, which it does very often. We love going away for the weekend in it with all our luggage and pram.
As for the front seats there's absolutely no comparison, the Ferrari's are more comfortable by an absolute country mile, In fact the whole interior is far superior and in different league than the maserati, the ferrari is all leather and the maserati is mostly man made plastics with leather seats - I suppose the list price reflects that.
I think my F1 box works brilliantly with the car, but you do have to get to know it, you can get super smooth gear changes using the paddles and backing off with your foot.. I also have the handling package which transforms the car from a luxury gt car into a snarling Ferrari at the press of a button - the 612 is the perfect car for me.