I've had a 2.0 and soon to get rid of a 2.5
The 2.5 isnt exactly fast as it eventually gets to 60 in 10 or so seconds, the 2.0 does it in a painstaking 12 or 13 from what i remember
I'd say avoid the 2.0 and a 2.2 at all costs, and an omega is quite heavy, and 2.0 and 2.2's only being a 4 pot, there just isnt enough get up and go (especially in 1st with an auto box from 0 mph) personally I would get th 2.6 or 3.0 if I was to get another one, i've also experienced a 2.5 (owned) and a 2.6 vectra and even though the bhp is the same, i noticed a massive difference in pulling power throughout the gears with a 2.6 with the torque being more.
I have never driven a 3.0 or 3.2, and I guess you have to remember tax and insurance and fuel economy
Why o why vauxhall didnt give the omega 5 speed + gear box I do not know, I just find with my 2.5 a higher ratio'd 1st gear would have made all the difference
Do vauxhall make 5 and 6 speed auto boxes now ?, i only drove a 2.0 tdi 140bhp 57 a6 the other day, and its got a 7 speed auto, from 0 to 60, i raced it with my omega, wiped the floor with a 4 pot 2.0 diesel

Get the 2.6 or 3.0 my freind, you will much appriciate more power throughout the rev range.