I have driven both, and quite a few other similar cars.
I would go for the BMW everytime over the Mondeo. It's a lot more refined, comfier and feels safer at high motorway speed. It's also worth checking out what comes standard on the 5 series is counted as extras on the Mondeo. One of my colleagues once 'loaded up' his Passat only to find out his extras were the same as standard on my A6 SE.
Skoda Octavia doesn't even figure in this conversation. Though the Superb might.
The new Passat is apparently lovely. Colleagues in Germany are choosing it over the A4/3 Series, but not the A6/5 series. BMW 2.0D engine is way better than my Audi and the Mondeo.
At autobahn speeds the Mondeo feels too light, German cars much more planted. If you have to lift off the gas when really trucking the steering goes washy and you realise you've driving too fast for the conditons in the Mondeo.
I've been shocked how tired and stiff I get in rental Mondeos compared to my own cars or other rentals.
Volvos do have the comfiest seat but seem to quite thirsty.
As mentioned above BMWs are surprisingly good on tax.
I went for the A6 SE as it came loaded (update arriving now) and also it's slightly easier to drive than the BMW- lighter clutch, gearbox and steering. Quite nice when you're are tired and aren't driving as smooth as you'd like. But I miss the old beemers and will most probably choose one next time