After completing now the 10k mark in my 991S Cab, there is not a single feature from the original order that I would be without. PSE gives the exhaust a totally different sound, I don't suppose with the weather that you had the chance to try one with and without with the roof down? Believe me, it's an awesome (as my 12 year old would say) experience.
PDK, having driven both for extended periods, the 7-Speed manual is clunky in the way it operates, it does however make the car 'feel' faster, because you are not on one constant acceleration curve. Putting the PDK into manual, and using the paddles is great fun, especially the auto blip on changedown. I also found the clutch on the 7-Speed to be heavier than that on the outgoing model.
3.4 vs 3.8 again, driven both, found that the 3.4 was nowhere near as eager as the outgoing 3.6 997, you had the wring the neck out of it as the powerband is much higher up the rev range.
2 vs 4 have not driven the 991 4, but purely from a widebody perspective this would appeal to me, plus with PDK in the wet, provoking the accelerator will cause you to drop from 6th to 2nd and light up the rear end as you found out, but you have to be provoking it with a sharp stick to do this.. and you can acheive the same effect with the 4 as I found out in a GTS4
My spec was as follows, but as all have said here, it's a big purchase, take your time, and take advantage of extended test drives, ie a weekend, and make sure that you try out different cars with different features to see which features you require.
Basalt Black
Black Cabriolet roof
Black leather interior
ParkAssist front and rear
Top tinted windscreen
Sports exhaust system
Sport Chrono Package Plus
Power steering plus
Porsche Doppelkupplung (PDK)
Porsche Dynamic Chassis Control (PDCC)
20-inch Carrera Classic wheel
Heated seats
Light design package
Floor mats
Guards Red seat belts
Cruise control
Sports seats plus
Interior Leather
SportDesign steering wheel
Preparation for mobile phone
BOSE® Surround Sound System
Automatically dimming interior and exterior mirrors with integrated rain sensor
PCM including navigation module