I haven't really read what everyone else has said here, but I came from a late model '02 (with tiptronics) to the 3 MT.
The difference in the tranmission probably has a lot to do with it, but overall I think that the design and features on the 3 surpass the P5 by a longshot.
The price gouging I got for being an early adopter in California (1-2 weeks on market) is still kicking me, but overall my monthly is only $30 extra.
If I had to do it again, I would still pick the 3 - it's just overall a more solid, fun to drive car. And a bit more practical in it's storage/cargo.
The only downside is that the rear seat can only be taken out, not folded up. And if you take it out, it takes about 45 minutes to figure out how to get it back in (seat belts do not go gentle into that goodnight).
Bear in mind - I've already have three seperate issues in my mazda 3 also... Not one in the P5, but I am much happier with the 3.