Zero has by far the better pacing. It's maybe 1 or 2 mission short but 5 dragged on way too much. I only played 4 after those so I found the plane handling weird. I liked 3 a lot (fantasy planes were great) back then, the campaign branching did work.
I also think Zero has the best visual environment, radio chat, it scales back the PNC allies (that really wasn't working), the enemy squadron system is awesome and the story is ok and much less grating than 5.
There's no denying that the formula was getting tired though. I would have liked PA to try improve air to ground, maybe ? But in truth I'm not sure that today's cutthroat market has a place for an arcade flying game.