Stop being a mentalist dude! Yours is worth more, and now people are so obsessed with petrol prices it'll be worth more in the future as a small car with an decent low tax derv engine.
If you must though, and I really wouldn't, at least get him to pay you the difference in values. And don't just go off Parkers or something, look on Autotrader and see what they're actually worth. His is going to run out of warranty before yours too, and it'll probably be showing rust in another year or two hehe
And as for the build quality, I'd say the Mk3 Clio was quite a bit better than the current Fiesta. Nicer materials in the Clio rather than the eggbox like stuff in the Fiesta.
Also, there is the running costs to consider, your tax is going to get expensive, driven equally it'll probably get about 2/3rds of the mpg yours will get, which is a shitload more than the difference between the cost of petrol an diesel...
What model DCi is yours? It seems like a silly swap to me, you'll be getting about the same car except owning it will cost you a s**t load more. But at the end of the day, it's up to you - if he's a proper mate he'll offer to pay at least the book value difference anyway. If he doesn't, kick him in the flaps.
Dan