I'm on my 2nd QP5, both Sport GTs with the semi-automatic gearbox. I've driven 2 recent Ghiblis, the diesel and the 410bhp V6 turbo S. I've more recently test driven a QP6 GTS, the V8 turbo with about 510bhp.
We all choose our Maseratis for different reasons, but for me, the key factors are driver involvement and sharp responses in a car that can take my family of 5 on continental holidays - blending my enjoyment of brisk driving with getting across countries in comfort with all our luggage. If I had most other sports cars, it would stay at home, and I'd end up driving a 2l diesel family car across Germany. That's just wrong. So now you know where I'm coming from, you can decide whether my comments are relevant to you or not...
Steering: All the recent Ghiblis and the QP6 have a slightly dead feel to the steering compared to the QP5 - verdict - QP5 wins easily on steering response and feel.
Acceleration and responsiveness: The diesel Ghibli works well with strong torque at mid-revs, not too much lag. The V6S is very strong, with more torque than my QP5, but there's little in it flat out. Out of these, I think I prefer my NA V8 4.2, where you just drop 2 gears to get lagless overtaking response. However, the QP6 V8 GTS is the king here, with massive acceleration on tap with no noticeable lag.
Combining the above, my personal preference for a driver's car is the QPV with duo select. The QP6 GTS can't quite make up for its steering and bloat with it's immense speed.
Refinement: The diesel is clattery at idle, but fine on the move. The V6S Ghibli was possibly a little more refined than my QPV. The QP6 V8 GTS is very refined and quiet inside in most modes. It has a valved exhaust which opens in Sport, which is subtly louder without causing offence - as far as I could tell from inside with the window down. It's turbocharged remember, so you're not going to get the same raw noise as a NA engine. The QP5 is very refined by absolute standards and compared to "ordinary" cars, but it isn't designed as a limo, it's a 4-door sports car, and its refinement at high cruising speeds (120-140mph, in Germany) is more than adequate, but probably not as good as luxo-barges. Outside Germany, where speed limits are 70mph-130kph (82mph) refinement differences are marginal. I think the QPV wins here if you're a sporty driver - you want to hear something without being deafened at speed on a long journey - its balance is right.
Practicality: I prefer the boot of the recent cars compared to the QPV, with its fuel tank behind the rear seats stealing space. We just about get everything we need in a QPV boot for a 2 week holiday, using the spare wheel well too, with a little thought and packing lighter than we'd prefer. So the massive QP6 boot is attractive. Even the Ghibli's would be an improvement. Recent cars win.
Economy: I think the recent turbo cars are better on fuel and tax, based on official figures.
Styling: QPV wins, easy
Interior: Not much between them.
Infotainment: The modern cars win. but who cares? Music sounds the same to me whether played off an SD card in my Passat or the CD changer in my QPV. I'm considering getting a Loudlink. Satnav - the QPV's isn't great by today's standards, so a Garmin or TomTom fills the gap. Phone integration - unimportant as mixing driving and phone calls is not something proper drivers tend to do. Clear win for recent cars.
My conclusion: I would definitely consider getting a QP6 (probably the GTS, though I wouldn't rule out even the diesel) at some point in the future, but there's nothing compelling me to make the change now, particularly as I don't like wasting money on heavy depreciation.