ST for me -- 5 pot soundtrack rules, handling pretty good out of the box, easy and reasonably reliable upgrades available if wanted and think will hold value better even though dearer on tax (confused on tax you quoted though as would the '06 2.5 ST not be 1080 tax?).
BTW... push hard for a discount as there will be few willing to foot the bill for running a 2.5 and don't be surprised to get close to a grand off that price level you indicated, or even a little over, if pushed hard on such a vehicle.
Negotiated on two different mid sized diesels this week and in each case got over 2 grand off on '08 cars - and would be popular cars unlike a 2.5 petrol - if you don't ask you don't get!
P.s. Ciaran is on the money too - go drive one of each and see what you like!