Go for it pete.... you won't be disappointed... however the GT doesn't have the same quality feel as the 159 does. Road noise is a bit louder in the GT and the dash is not as stylish as the 159. The GT cabin is a nice place to be though, the seats really do hug you.
As for spec try look for a GT Monza JTDm (or 'Blackline' if a UK import). This has a nicer spec with 18" stylish wheels, some minor tweaks to the exterior and interior trim. The Cloverleaf is also a sweet spec too. The only downside is that the bigger wheels result in a more thrashy ride.
If you want to have a look over my GT the next time I'm down in LK your more than welcome- mine is a petrol though.