Follow RichW's good advice - test them all and decide yourself! As for my personal opinion...
Firstly... I would only get a diesel if mileage was a huge factor. My 1.8T was getting mid 40's mpg on motorway runs, which is absolutely fine for me. If anybody wants more than that, they aren't serious about wanting a hot hatch - diesels excluded obviously, but then I'm not a rep pulling 30,000 miles a year for example.
Secondly... I owned a 1.8T for years and absolutely loved it - It was the perfect compromised hot hatch, in that it was plenty quick enough when required, but very smooth, very quiet, excellent sound [factory] system, lovely seats, amazing cabin quality, etc. etc. The Mk4 gets a lot of stick... Yes its not as fast as a Focus ST, but then how often are you doing 0-60 runs? Yes it doesn't handle as well as a hot Megane, but then its not as crashy or poorly made either.
You'll love the Mk4!