I don't have a P5, but I do have a Protege of the same era, so the build quality and front interior room comment observations will apply. The Protege has better interior materials--seat covers, carpet and panels are thicker. Both of them have hard, cheap dashboards. It's occurred to me that Mazda had to do a little de-contenting to get the price of the 5 down. The assembly quality of both are fine, though. The 5 does have a lighted driver window switch and steering wheel controls, both of which are missing on the Protege.
Something else you may notice--the Protege has more room in the footwells. The 5 actually borders on tight for me. It may be that the Protege has a surprising amount of footwell room for a car in its class size. The driving positions are very different as well; when I switch from one car to the other, my left foot gets confused as to which way the clutch pedal is going to move. I'm betting you'd find the 5 smoother and quieter, as you'd expect for a newer generation vehicle.
No comment about the engine performance since I have the LX and the 1.6 L 105 hp engine. The 5 beats the snot out of that Protege.
No comparison on the cargo room. I know the P5 is on the tight side in the back for a compact wagon. By comparison, the 5 is cavernous.