Humm this is tough.... I own a facelift Omega and a Senator.
Unfortunately I do not have a big 3.0 V6 Omega just the 2.2, so comparing the them is unfair as the Senator as a HUGE power advantage.
It all depends on your budget? If you got £500 quid or similar, I'd prob get a Senator. But if you got more like £1,000 to spend, then i'd find a post mini facelift (98 onwards Omega)
The straight 6 in the Senator is brilliant, you must know from ownership. Its fast and produces a lot of torque.... quicker than the retail 3.2 V6 Elite...
However I have to disagree on a few posts above:
The Senator to me is MORE supportive and comfortable than my Omega! The seats are just so much softer ... on a 70 mile run to Uni i felt more refreshed from driving the Senator than Omega...
Of course the Senator is getting on, there is no doubt the Omega is a better car in terms of safety handling (on the limit).. looks better, rust is not a real isssue....
However as Laidback commented, there is "something" about them .....