I have a 1.9dti Laguna Estate and it's geat on mpg. I always get 50 mpg no matter where I'm driving, town, motorway, etc. Best I've done is 58 mpg on country roads were you're doing around 50mph most of the time. I do try to drive smoothly to maximise mpg rather than thrashing it everywhere though.
Although I've found you don't need to rev it like a petrol engine anyway as there is plenty of low down torque. My revs are normally between 2000 and 2600 rpm, which I would imagine is why my mpg is pretty constant.
I've done two return trips to North Wales over the Xmas / New Year period with 4 or 5 people and a boot full of luggage, and still got 50mpg at constant motorway speeds.
I've have a read of a few threads on here ragarding the DCI engine, seems there can be a lot of problems with them,
failure for one. Personally, I'd go for the dti, I think the mpg figures are quoted as slightly better for the dti too. Thing is you'll be looking at an older car as the dti finished about 2000 - 2001 I think.