I think it all depends on what you are looking for. The GN takes some getting used to. I mean it has some of the best and worst engineering of the time period. Friends I take for ride who have 'modern' performance cars love (and I do mean love) the look and acceleration my car has, but almost all of them comment on the squeaks, rattles, wind noise (when windows are closed) that my car makes. Coming from a mustang you also might be used to a different level of, amenities as well (You know, things like cup holders haha).
If you are looking for a power rush, the TR's can for sure provide it, but as 626 mentioned, it can take patience to 'dial one in' and buying someone elses headache can spoil your taste for our cars (and like he said there goes your wallet). I encourage you too hook up the Turbo Buick community in your area. I think we universally never get tired of talking about our cars and we are not shy about telling you straight up what our experiences have been. Ultimately they will be your best source for getting your new purchase sorted out and dialing it in. Getting to know them ahead of time could help you 'dial in' whether or not you would enjoy one better than your Mustang.
If you find one you are thinking of getting, feel free to post the pics and details here. It is amazing to me what guys can tell off of a single pic of an engine bay. They can tell you some visual indicators of things you should look closer at and get you armed with questions for the seller. TAKE YOUR TIME in making your selection. The biggest mistake people make is jumping at the first GN that catches their eye thinking they will never find another one. I looked for almost 3 years before i bought mine, and it has been everything I had hoped for.
Good luck in your decision!
Either way it's a great "problem" to have 
