I'd go for Japanese every time. We've just said goodbye to a wonderful Corolla that did 240 000 kms without a single thing ever going wrong with it, despite the fact that we'd never serviced it (beyond one or two oil changes). Similar story for my parents-in-law - they had a Picnic which they thoroughly neglected yet which remained reliable until 250 000 kms, when the belt eventually snapped (they'd never changed it). Now we've got a Prius, one of the most reliable cars on the road (but I suspect that this is the other end of the spectrum to the sort of vehicle you're looking for!).
About diesels: over here (and in Germany too I believe) they're the most popular engines by a long margin so I wouldn't worry about how good they are. They're definitely more reliable and economical, but they pollute more than petrol engines.
I think that Mazda is a great vehicle, though. I remember seeing Richard Hammond waxing lyrical about them on Top Gear once - apparently they have a great gear box. As for the Alfa, one of my colleagues has had two. Lovely to drive and they look gorgeous, but she's had quite a few reliability problems with them - she's now thinking of getting a hybrid like mine!