I agree with the people who say it isn't the film.
I liked Plux-X, but one of the things I liked about it was how well it worked in Diafine. I could shoot it at box speed in D76 (or whatever) or, if needed, at 400 and develop in Diafine, and carry one film. None of the other medium speed films I've tried have responded as well, and that includes FP4+ which does get an effective boost, just not as much. People who have tried Shanghai GP3 in Diafine say it is definitely not Plus-X, in this way at least.
But that's fairly minor. I have good 400 films I can use at box speed any time I want. I switched to FP4+ for medium speed (have a few rolls of Acros to try though) and like it a lot. The differences between the two, while probably not non-existent, are simply going to be drowned out in the noise compared to the differences in photographers.
I have prints in the same series that are from FP4+ and Plus-X negatives. I dare anyone to tell them apart.
Some of my series span longer periods typically 25 years+ and I have no issuse putting prints from Tmax100 and AP/APX100 alongside those shot more recently with Delta 100 and even HP5 (LF) although I have had to reprint on current papers but that's another issue.
I've been using more FP 4 recently and sure like it. It will most likely become my replacement for Plus-X. I get excellent results out of either and can't say one is better than the other, at least at my stage in the game.
If you are talking about trying to use APX 25 instead of TMZ, yes, it's the film. If you are comparing Portra 400 with Velvia 50, yes it's the film. But between Plus-X and FP4+? I'm with Thomas on this - develop both to the same contrast and I'd defy anyone to tell the difference in the prints by eye.
There is no "best". It all depends on what you are doing and what look you want. Plus X Pan was
appropriately marketed mainly as a studio portrait film. It had a very long sweeping toe designed to
give subtle midtones and highlight gradation. If you had a soft lighting ratio you would get gradation
in the shadows too. But with strong lighting you'd have to resort to compensating dev to dig deep,
and that would likely spoil the delicacy of the uppers. Not a very good film for direct sunlight conditions with deep shadows. The closest thing today in curve shape is Delta 100. FP4 has a much longer straight line, as does ACROS, though neither are true straight line films.