both are great cars, i have one of each
£1000 will buy you an early phase 1 GTV 2.0, be aware that there is a lot of dross out there currently, cars that have been badly serviced or have no had any work done on them for a while, as a result a £1000 car can easily turn to a money pit...
Cambelts service intervals are key, make sure it is not more than 35,000 miles or 3 years, if it is then factor that into the purchase.
Rear suspension on the GTV can be troublesome as well, if it has been powerflexed then that will eliminate this as a worry.....
The drivers and passengers window glasses have a tendancy not to seal too well against the rubber seal and so you get whistling noises, this can be a real pain in the a**e to sort out
The 156's issues mainly surround the front suspension, listen for squeaks and knocks, cambelts are again something to watch on them.
If you have a family then the GTV may not be the most practical car in the world, the 156 would be the better bet, if it just the two of you then out of the two i would go GTV everyday.
Driving wise the GTV is a little more 'edgy' that the 156, but that said the 156 is still pretty sharp considering it is a family saloon