As far as I am aware the V6 engine is alot more durable in that (Being a larger engine) it can take quite a few more miles, and can take fewer cam-belt changes over time. (TS engines require changing every 36K miles / 3 years, whereas the V6 engines require changing every 72K miles / 5 years)
Servicing wise, the T.Spark of course has 2 more spark plugs than the V6, so there's another cost.
Performance wise, the V6 would obliterate the TS on a straight, but take it to the twisties and the T.Spark may gain some ground.
If you can afford the fuel, insurance etc I say go with the V6, if not for the sound of the engine alone.
I have a 2.0TS and it does lack a slight punch of power that my 156 V6 used to pack, and that was 'only' 190BHP, let alone the 220BHP the V6 GTV kicks out!
Do bear in mind though the V6's are quite a bit more expensive, say around £3000+, whereas a decent T.Spark can be picked up for as little as £1500 in good condition. But mine's not for sale,