Important thing is to test drive a diesel and a petrol from whatever model you like...
There are good diesels and bad diesels, likewise there are good petrols and bad petrols.
From driving various hirecars, I prefer the BMW diesels to the VAG diesels: the VAG diesels are brilliant but the BMW ones are even better.
If you are used to petrol engines and do country lane driving, you need to be wary of one point of diesel engines: they feel wonderfully potent during normal driving, but then you floor it to overtake someone you find you've been using 80-90% of your torque all along. Pedal-to-the-metal isn't quite what you were expecting. Plus there is a fraction of a second's delay between pressing the accelerator and getting some oomph (not turbo lag, just down to the passive nature of diesel induction), so it doesn't feel as "tight" as a petrol engine.
If you're interested in the actual ability of the car rather than just the *feel* of the car, take a stopwatch too: diesels are very peaky, so can feel mighty but when compared to the more linear torque of a petrol engine are actually slower, despite *feeling* faster. I drove a BMW 120i diesel that *felt* faster than my own car, but when I timed it it was 2 seconds slower from 70-100mph.
I'd definitely consider a diesel for my next car, though. A 330 or 335 would do me just fine