Unfortunately most of the replies seem very biased, which is not surprising given this is an Omega forum. I own a 2006 ST220 and previously had a 3.2 omega. The miggy was older 2003/53 and had 60k on the clock. Auto in elite spec and serviced and maintained very well by its prev owner. I only had the omega for 6 months due to its drinking problem. The ST on the other hand was reasonably economical if driven sedately, but when driven hard it had the edge over the Omega most if not all of the time. Felt much quicker too although comparing a manual to an auto is not fair. What the ST lacks is the comfy ride of the Vx over our beautifully surfaced roads.
Although most people inc me discount FWD cars, the ST was a surprisingly good car to drive. In fact as a drivers car I preferred the ST to the omega especially having taken both to Brands Hatch. The elite went because I did not see the point of such a thirsty engine in what was to me a relaxed cruiser. I do however have a 2.2 sport which I still have because its economical and yet comfy.
So from an owners perspective if you wanted a greater focus on comfort go for the Vx, as a drivers car the ST is much better. All IMHO.