I would argue a much simpler rule:
Will this be your daily driver? If Yes, then the 2.0TS everytime - which is what I run in my 156 Veloce, as need some practicality, i.e. a usable boot
The twinny is great, really light weight by comparison and very chuckable in the bends.
If this is purely a toy / weekend runner, then you have to have the V6
just purely for the noise. BTW, it will be heavy on fuel if you have a heavy right foot
but an addiction that is well worth it.
You will notice the sheer grunt of the thing and the torque is tremendous, even in 6th gear you can just press on with it.
The Spider version is more twitchy to the GTV as the rigidity % comparison is shocking. Most people fit strut braces to compensate. But, again if its a summer toy then a Spider is very nice all the same.
Whatever you end up with, you will find everything here on AO.
All the best in your quest.... you won't regret it
PS. Forgot to add, you will want a Pre-2001 model (before March) in order to get the lower RFL. It's great taxing a V6 for £200pa
I personally prefer the Phase 2 revision (I think this was 1998>) then you got the later PH3 with the drop nose (bit Marmite IMO). The interior is drop dead amazing and quite Ferrari-esque, same to boot with the V6, having been developed by Mr. Busso who was also the Ferrari V8 man, so good DNA as it were.