My experience of 156's goes something like this:
w reg 156 SP CF2 V6 - 240 to 270 miles per tank. Owned for 18 months, one breakdown - MAF
w reg 156 SP JTD (re-mapped) - 270 to 300 miles per tank. Owned for 6 months, no breakdowns but expensive repairs were looming
x reg 156 SP CF3 2.0 Selespeed - 270 to 300 miles per tank. Driven for 6 months, owned for 2 years, no problems but it had a particularly good engine.
52 reg 156 SP CF3 V6 - 300 to 350 miles per tank. Current car, bought with a broken belt and rebuilt. No problems.....yet!
I'll be the first to admit that I don't drive the CF3 v6 as hard as any of the others but even being a loon I'll still get more than 270 miles out of a tank. All the figures were obtained driving 8 miles each way to work with probably a 20 mile run during work time, all on a roads, b roads and urban environments.
The 2.0 is only significantly better if you do motorway runs where I'd get 450 odd miles out of a tank and only 350 out of the CF3 v6.
I think the key is the CF3 v6's have more sophisticated engine management and can give better mpg as a result.
I won't deny that the 2.0 handles better, it certainly has better turn in and a more oversteer biased feel but the v6 can be sharpened up easily enough.
2.5 v6's seem to be okay up to 60k odd miles before needing a cambelt, I wouldn't trust a twin spark pask 40k though. Remember there are also age limitations relating to cambelts when buying...
The V6 really needs to be worked hard to get it to perform, its a bit guttless below 4000rpm but once there, they are genuinely quicker than a 2.0, but below 4000rpm I'd be half inclined to say the CF3 2.0's may even have the advantage. If you're not a rev head, get a 2.0.
With regard to reliability, I think the v6 has the edge over a t-spark but when they go wrong, they are more expensive to fix in my experience.
Personally, I love my V6.