Have you driven a 320d? I think you might be surprised in that the motor is very strong. Might not be the most exciting but I suppose that's the compromise. If you want BIK and MPG at the top of the priority list, excitement will automatically fall down the ladder.
At least with the BMW you'll get the balance of a rwd chassis, the 320d is certainly powerful enough to adjust the cornering attitude on the throttle. You'll not be pulling big drifts but certainly a little shimmer on a wet roundabout makes things a bit more fun!
The g/f has a 320d M-Sport Touring. It is a worthy, if slightly dull car, handles quite well. The Sport-Auto box is excellent. The engine is OK, a little dull... not as good as the engine in the GTD... but not far off - it's more old school diesel.
If it were me, I'd pay a little more income tax and go for the 330i, on our scheme there's barely any difference in monthly lease cost. Yes, you'll lose out a bit on fuel consumption but performance and a petrol engine, for me, would make it worthwhile. BTW, I know it is expensive but the M-Sport + pack really is the way to go. Forget the frivolous options like keyless and reversing cameras... The M-Sport + pack has pretty mucg everything you need to make the car as it should be.
As for the Merc 350e, on our scheme, for some reason, it is almost 50% more than the BMW 330i... and on our scheme I'd have to pay that... and the tax saving won't offset that.
I'm not due to change mine for another couple of years... And it is difficult as the tax regime is likely to change to penalise the latest "bad thing"... They've done it before. Who knows, in 2 years there may be plug-in hybrids with decent range...
Talking of plug-in hybrids, they work quite well if you live in a city. A colleague has had his Golf GTE for a few months. He reckons that he's only filled it up with petrol a couple of times... He just swans around London on e-power and then charges it up overnight for the next day... But for me it would be useless, I rarely do journeys of less than 100 miles when I'm traveling for work.