To Davo5X7 -
Most folks in my experience prefer the straight Tri-x (400) over TXP (320). I was told once that TXP was intended for "studio" use and had better response to incandescent lighting, or something like that-the memory fades. I used the TXP320 in MF many years ago, and always felt that TX400 had better crispness and tonal distribution outdoors, but not from any scientific testing.
Anyway, my default speed for Tri-x (400) is 250, mostly from experience with Rodinal, HC110 and TMAX developer, in 35, MF, and 4x5. Whenever I test it with a developer, I always start there and bracket test exposures from that point, which I am currently doing with PMK (Pyro from bostick-sullivan.com), and like it so far.