Hi,

I honestly think you can use either of those films. I have tried them all and like them all. They are all wonderful films. My pick would be APX100, because it's such a good all-round film. It can be developed to achieve a small enough grain to make very large prints, and it can also be pushed several stops for speed.
Pan-F+ is ISO50 - and its disadvantage is that it's ISO50 - too slow for some. Tri-X arguably has a larger grain than the others, which can be of disadvantage. Both APX100 and FP4+ are good all-round medium-speed films, and it's a matter of personal taste which you prefer.

My advice is to start using any of those films, but pick one. Use that film exclusively. Get to know its qualities, its weaknesses and strengths. When you're illuminated with an in-depth knowledge of the film you like to use, your results will be consistent. You will also have a much easier time printing, because you know better what to expect.

There is one down-side to this approach. If all of a sudden the film you use would be withdrawn from the market you have to go learn a new film again.

Anyway, bottom line is - Pick one film and stick with it. You can get wonderful results with either of the films mentioned.

Hope that helps,

- Thomas

Saint Paul, MN