My 911 is much like yours is, or was - 1987 in Marine blue, though mine is RoW and not the wide body you made yours. A couple years back I almost switched to a 993. It was purchased just an hour or two before I could get a deposit in on it. I have decided to keep my '87 and am happy I still have her but I still think of that sweet 993 from time to time. Yours is a somewhat enviable position to be in; I wouldn't mind at all to have a GT3 with my name on it. Would I sell my 911 to get one? Maybe. It's tough to say for sure without being in that position.
I'm just spit balling, but here's a couple of thoughts...
I mostly enjoy working on and tinkering with mine. With the GT3, you likely won't be able to do that as much, if that sort of thing floats your boat.
Your 911 was never the top of the performance heap of its time, and the GT3 is.
Is your 911 something that would break your heart if you lost it forever?
A lot of this boils down to emotion. The extra cost of the GT3 won't usually matter. I mean, really, none of us bought ours based more than a tiny bit on logic.
If the GT3 makes you giddy and plasters a big ass grin on your face, I think the choice is obvious.
EDIT: the other thing I never considered: if you get the GT3, none of us will wave any more because you're driving a water pumper and everyone knows they're too pretentious and never wave back...