Very different beast to the M3 Joe. I've had the s54 engine in an M3 CSL and a z4m coupe and the older one on 2 x z3m coupes. Personally, much as the CSL is revered, I found the z4m to be the better car, especially once subtly modded with CSL wheels, better suspension and brakes, GruppeM airbox, Supersprint etc. I loved mine and felt it to be a more special car than the CSL. I know i'm probably in the minority with that opinion!
The GS power delivery is different, with oodles more torque making accelleration feel more effortless than brutal. It's a special car and one that you will look back at and smile when you park it and one that you'll pop into the garage to look at when the weather is bad. The reaction from others is always good, people have a real respect for Maserati and will think you are a classy guy to own one !!
My advice after having owned two is this - low mileage is not necessarily a good thing and low mileage cars will cost more. They like to be used, not left sitting. for me, 35-40k miles at least and don't be afraid of a 65-70k miler if the price is strong. Black, grey, any blues are most desirable although just about any colour is good. Red has become popular and white is the king but you'll never find one. Some don't like silver but that's because it's hard to photograph well and actually is looks great in the flesh. This is a good tip as silver cars can be keenly priced. Yellow is horrible (that's for you MAF!!!) but don't worry as there are only 2 and not for sale !! Desirable options are LE spec which is full leather, xenons, upgraded (but still ****) hifi, heated seats, park sensors. Some have strong opinions on must have options, I don't agree generally and I would always buy the best car based on history and condition, not whether it has cruise or polished wheels.
Any one you want to buy, get it checked over if it's not from Dicky, Alastair, Nareman or someone like Giallo. Even if it is from them, it's not bad to still have it checked. The engines are strong as are the gearboxes but suspension can wear and parts can be expensive. There are known weak spots but nothing too dramatic. Clutches are known to fail but not as much as you'd think. You can attain a reading from them but it's not an exact science and shouldn't be relied on. A car with a recent clutch (inside 2 years) is worth seeking out if possible. They should last 40-50k miles unless driven in auto regularly or abused.
The SMGII box is not better or worse, just different. The CC box will seem agricultural at first in comparison but once you learn to take it by the scruff of the neck and be a little more bold, you will find it to be excellent. Settings can be adjusted, get friendly with you local independent and play with it once you're ready.
Good luck with the hunt.