The 2.5 / 2.6 is reckoned to be the best balance of performance and economy. If you're going to be doing a lot of urban / town driving a manual will be significantly more economical and quicker and the auto gearboxes can give trouble. Other than that reliability probably isn't significantly different across the range. The engines are practically identical apart from bore, stroke and cams being different.
The 2.6 and 3.2 models do away with the EGR and air injection systems which can be problematic and also the plug leads are eliminated by the coil-per-plug ignition system but they have a drive by wire throttle which, if problems occur, can be expensive to fix. On balance not a lot to choose between them.
Then again, the official fuel consumption figures are only a couple of MPG different between 2.5/2.6 and 3.0/3.2 which is why I went for the latter, which has a good bit more power.
I get about 27-28 MPG on my commute to work (single carriageway B roads with a little urban). It'll do 30 MPG on a run at fast motorway cruising speeds and if you stick to 70 I have seen up to 35 MPG. This is from a 3.2 MV6 auto.
Kevin