Subjectively, the GTV will feel faster than the 156, as the bigger car tend to "cosset" you more. There is more sound proofing in the 156, which (imo) dulls the sound of the engine. I have driven both the 2.0TS GTV and 2.0TS 156, and the GTV was much more fun to drive (although I still love the 156 more
I don't think there be so much of a performance hit on the 1.8TS GTV that would make it any less desireable.
johncc - Did the bonnet need replacement in the end? If so, check out the prices for the replacement and use this as a negotiation point.
Also, find out when the cambelt was last changed. Remember that on the TS engines they need to be changed every 3 years or 36k miles. If there is a diesel sound on start-up that means that the variator probably needs doing at the same time.
Finally, make sure the previous owner knows where the oil filler cap is, and has used it on a regular basis.
Good luck with your purchase, and let us know how it goes.
(btw, if you do buy it, make sure you post some pics here too