If you want a car capable of overtaking, I'd suggest you avoid the Rover 75 CDT (or CDTi). It's a BIG heavy lardy lump, making it a rubbish "sports saloon". Having said that, has a great interior and is definitely a "step up" inside when compared to other named competition. It also eats miles, and is very comfortable and relaxing.
I tried the Mazda6 diesel whilst on holiday in Italy. Almost a sports saloon with a very perky engine. The interior isn't as good as the Rover, but it makes up for it by being a good deal lighter and does handle well. Definitely got my thumbs up.
I'm guessing the mk3 TDCi will be a very good car. Tried a petrol version and was impressed by everything bar the engine. With a good diesel, would be a great allrounder, and IMO sits somewhere between the Mazda6 and 75.
It's really a question of preference, none of them are bad cars. The Mazda is the most sporty and feels the lightest. The Rover is more relaxed and the least sporty, the Mondeo sits pretty much in the middle.