Just some thoughts for you, I'm no expert on GTV's but;
I Doubt a GTV will be as quick as a 306 GTi6, the 2.0TS is a gem of a motor but it's not a rocket ship.
Economy won't be bad though, probably high twenties.
Course you can use it everyday...why not?, unless you want to carry more than one Passenger, then it helps if they have no legs.
Rot is NOT an issue, hasn't been on Alfa's for years now.
GTV's do have a few known problems, but not that many, do a search on this site, for the GTV/Spider Buyers guide, it was thread started about 6 weeks ago, to help new or potential owners.
Most people who have GTV's love them, but some also have a bit of a love hate reliationship going on...
I would be suprised if Parts for Fiat Coupe's were easier to get than a GTV, why do you think that?
The Fiat coupe has a lot of respect on this site, they are very fast, handle well and the quirky looks are easily Chris Bangle's best design, though that's not saying much TBH
The 5 Pot motor has to be completely removed to change the Cam Belt, which is a bit costly. I would have thought the Turbo would be expensive to run with fuel and insurance etc.
I suspect it will come down to which one gives you the best wow factor when you drive it.
Good luck, and welcome to AO BTW