I'm not yet a 991 owner, but will hopefully have my 991 Carrera before Christmas.
I avoided the sport seats as I found the "wings" encroached on my shoulders and gave me backache. (or at least that's what happened when testing 997 with sport seats) I did go for heated but avoided cooled seats. Had these on BMW and never used them (they also cause lower back ache!) so cost avoided. I went for SportChrono although not initially - I didn't buy the car to sell it however I am mindful of residuals and I "think" I'll see some benefit with dynamic engine mounts. (and quite like the clock in 991) It's different to the 997 in terms of it's necessity IMO. Also went for parking sensors front & rear. Whilst the 991 is a smallish car I often use a car park which is fairly tight so have found it useful on 997. I’ve caught myself reversing into spaces because I have parking sensors! PDK is a must - the core reason for me upgrading. Colour - all subjective however the Platinum Silver isn't as nice as Arctic Silver with silver wheels - (and I'm not a fan of painted wheels) so stuck with the classic black\black combo as I simply love it! Basalt seems to age better than the solid black and I’ve found most Porsche solid colours seem to oxidise slightly over time. (especially guards red)
Bose - it's much improved from the 997 but I would have foregone this had pricing been the biggest issue. Mobile phone prep - a no-brainer in the UK and 3rd year warranty was a deal sweetener. I also went for TPMS after taking on-board comments from fellow posters on the Porsche implementation (again, found it useless in BMW) and hopefully I’ll be impressed.
I perhaps didn't saviour the ordering process as I did when I last spec'd a 911 as I'm much less frivolous this time round and although I wasn’t forced into the purchase, I hadn’t actually intended to buy on the day I test drove it! My 997 is very well spec'd but thankfully the options were paid for by the original purchaser.
I was looking at used 991 for a while before taking the plunge but I was surprised at the incentive offered to buy new. The car I’ve ordered is similarly priced to the 996 C4S I purchased new back in 2003. It has much more kit and is faster and much much much more comfortable and yet also more sporting. Wonder what the price of 991 would be if it kept in line with inflation?
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C2S - Basalt/Black XC90 - Black/Black 2 kids, 1 dog
Anyone with back issues will appreciate the lumbar support adjustment. 
