Thanks guys for all the replies. Lots of conflicting advice, but hey what did I expect... 😉
Toby, you have a point when you write that "picking two films from the same family might make them easier to match up if you make a series of photos to display together". Actually, my interest in Plus-X was triggered by a body of work Raymond Depardon (of Magnum Photos) made in Africa over a period of several years using both Tri-X and Plus-X. The whole series struck me as quite uniform in tones with all the photos having a very nice, rustic look to them, including those taken in very contrasty light.
Robert, I do rate Tri-X at 200 when shooting in bright sunlight, however I would have hoped to get more dynamic range by using Plus-X instead. But you're right, assuming there's something to be gained in this respect it is probably not significant enough to justify having to "learn" a new film. Still, I'm tempted to follow William's advice and give a try to Plus-X rated at iso 125 and developed in D76. We'll see...
Thanks again.
Vincent