Go and drive both of them. The biggest difference is driving dynamics - the signum is pretty poor and the Mondeo is especially good. But the signum will probably give you something newer and higher specced for the money - so entirely depends what's important to you.
Just don't buy a diesel, it will cost you more in the long run!
Have driven a few Mondeo's but not a Signum. Guess I need to. I agree you tend to get more car with the Signum. Will not be looking at the diesels then
