Which Mazda 3 should I buy?

Generally, I'd say the highest spec model you can afford. However, in the Mazda3's case I'd actually say the Maxx seems to provide the best bang-for-your-buck over the far more expensive SP25 variants.

It will come down to whether you find the hatch or sedan more practical, and whether you can live without the extra goodies on the higher-spec models. If you're doing a lot of country driving, I would give extra consideration to the 2.5-litre SP25 models as the 2.0-litre is fine for around town but a bit breathless on the open highway.