People tend to say 2.0L is a good balance of performance and efficiency. I'd agree
1.8L and 2.0L are practically the same in terms of MPG, so you may as well get the 2.0L and the 1.6L is considered a bit slow and also not the same Mazda MZR engine, seen as reliable and also shared with Ford Focus, Mazda 3, Volvo S40/C30/V50 etc so as previously said, easy to get parts for. Also there's no cam belt, so one less thing to worry about.
Mine has 137K on the clock and is still going strong (had a TerraClean about 1.5 years ago but didn't really make much difference to be honest).
No engine problems in my ownership of 3.5 years now