Well!! What shall I say!
As you can see I own both...
Firstly I think the Comparison is unfair ... The Omega is a 1994 design, the Senator/Carlton is a 80's design so of course the Omega is going to be better...
SenatorBigger, heavier than Omega, longer bonnet, the big Straight 6 up front has more space around it, compared to the V6 in Omega. (easier to work on generally)
I find the Senator more comfy than the Omega, but the Omega seats are more supportive if that makes sense! :-?
The Electrically Adjustable suspension on the Senator is fun, its a completely different car with the suspension set on Comfort, handles like a P&O Ferry. Sport and medium is good, handles as well as the Omega, unless right on the limit (i'll let MarkDTM explain further

)
OmegaMy Omega (being a facelift) rides a
LOT better than the Senny, also is silent at speed. The Senator has a lot of wind noise @ 85+
Omega looks better has more toys.... from what I can tell its just as well built as the Senny. My Senator is 15 years old 140k miles, still holding together inside very well.
Body work is crap though on the Senator, very thin metal and rusts easily!
Can't really compare performance, as I cannot give a fair review. Omega is only a 2.2 Senator is 3.0 24v...
However I have driven a well maintained MV6 3.0 24v, the Omega feels slower due to the multi-ram, with the Senator at 4k rpm you get a HUGE surge.... on Comfort mode the car's bonnet goes up and bam your off!!!
But as TheBoy mentioned on a little test to 1 *cough* 3 *cough* 0 ish mph

- The MV6 and Senator were neck and neck, with the MV6 nudging ahead, but a very small amount!
It really was very close, and impressive from a 15 year old car that cost £300!
Ohh and the digi dash is cool on the Senator!
