You can use TXP outdoors, but you've got to be a bit more careful to give sufficient exposure to get the shadows off the toe of the film curve (i.e., to get good shadow separation), and to control development, particularly when you've got very contrasty lighting, or the highlights will be difficult to print. I use it all the time.
John, if I were you I'd buy a roll of each and shoot an identical scene with both rolls, develop them, and see if there is enough between them, in practical matters and for your purposes, to worry about.
The 320 isn't as bad as you might think. It's great film, and you'll enjoy both if you learn how to use them to your liking.
Don't think. Do! And take notes.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh
Another important thing is that the TXP 320 is, as far as I know, the only
black and white film available in 220 size. I have used it during the last
30 years for general purpose, and as long as you have a good lighting situation it gives excelent results. I normaly develop it with D-76 or
HC-110 and they both work fine. If you develop in tanks with film spirals ( such as Paterson ) , be carefull so the spirals are bone dry when you load the film. The 220 film is more sensitive to jaming when you load it as it is
twice the length of a 120-film. With a 220-back for , in my case , the Hasselblad it is a great feeling to be able to shoot a rapid sequence of
24 2 1/4 square frames without having to reload.
Give it a try. It is worth it !
Godd Luck !