I'd say take them all for a decent test drive and see which one works best for you. They may look pretty similar but they're very different to drive! I've tried both of the 997 cars and found the GTS to be the most thrilling of the two, however what put me off buying (in that instance) was the PDK. Not that the gearbox isn't excellent, it's just that I felt it took a bit away from the driving experience. Also, it'll depend on where and how you intend to drive the thing. The 4s is AWD and therefore easy to drive in all conditions, whereas the GTS is RWD, which is great if you want to throw it around a track but maybe a bit of a handful on the back roads.
I think the other point to consider is depreciation (and this is relevant to your 991 question as well). Both the GTS and the 991 are relatively early in their lifespan and therefore have a bit to loose before they start to tail off. Depending on the age of your C4S, you may find that it doesn't have as far to drop before things even out. Of course, if you intend to run the thing till it's dead none of this matters!
Having said all of this, I don't currently own any of these three cars. Maybe some of the folks in here that do can provide more in depth commentary on the day-to-day experience. I do note that there seems to be a relatively large number of low mileage GTS for sale.