Well for me this played out differently. I currently have a 997.2 Targa and the Cayenne for the wife. The Targa has been the greatest DD anyone could ask for. In the winter is was solid as ever (4S) in the summer the back hatch gave me plenty of room to throw in my clubs and other stuff. A true DD with fun to boot - i will miss it. Porsche even gave me a 6k incentive to get out of my car and into a new 991 (called a pull through). After testing the new 991 I would go so far as to say that its more comfortable and easier to live with them my 997. However a few things pulled me to the M5. The 1st was our newborn in the house - my wife needs the Cayenne almost everyday and there are days (key ones) when I need a car that can take 3+ and blend in at customer sites. The 2nd was timing, I really wanted the C4S with electric glass roof and Porsche kept delaying the launch and withholding the glass roof option until last week!!! 3rd was change, having the Cayenne and the 997.2 I wanted to get back into a BMW with all the fun gadgets like soft close doors, HUD, NAV, etc. Finally was price, the cheapest 911 I could live with (configure) was around 160k and thats about 35k more then the M5. With so much change at home and not knowing what model of the 991 I really wanted (C2S, C4S, GT3?) I went with the safe best and got an M5.
Now in all honesty the practical side of me should have gone with a 335x or an S4 to make room for the 911. BUT after driving the M5 there was no way I was not getting one - especially in MT (the last of its bread). Will I miss having a 911, yes, there is absolutely no substitute and I have warned the wife that I will have to add a 3rd car to the stable. Will I love the M5, yes 100% Its a great city driving and cruising car with speed and fun to boot.
In your shoes? I would go 991 - test one and see how you feel - its going to a great ride a fantastic DD. in my book the M5 and the 991 Targa/GT3 are the two cars to have in the stable.