Diesel or petrol? Completely depends on the car tbh... i.e Ford have just released a 1 litre ecoboost engine (petrol) with something like 125bhp... not bad when you consider it does 55mpg in a focus. I think these smaller petrol engines will be the way a lot of manufactures go in the next few years.
Diesel or petrol? servicing costs vary widelt from garage to garage. if you have a mate mechanic then the cost is negligable. also if buying new barter for free servicing.