1. TZ provides a "vintage watch" and a "watchmaking and repair" forum.
2. TZ has more traffic.
3. TZ has a lot more lower-end pieces in its sales fora, as well as straps, winders, cases, etc.
4. TZ has overt commercial sponsorship, including a very close relationship with Antiquorum, so there are more obvious "synergies" with commercial sites than are found here. I do not find that objectionable, and TZ so far seems to have handled the inherent conflicts well.
5. Some of TZ's moderators can be a bit idiosyncratic (not that ours aren't, just that ours are always politely idiosyncratic). On certain fora the moderator's mood seems to be a major determinant of activity.
Subjectively:
a. I like and use them both. My appetite for watch content exceeds either site's ability to produce it.
b. I tend to think of PuristsPro as providing a more consistently high level of insight. TZ has some great posts, but many that I find much less interesting.
c. I am primarily a vintage collector who takes a great interest in watch restoration and maintenance. PuristsPro is strongly oriented toward those who buy new from ADs and then sent their watches to the factory for maintenance. Thus, TZ provides some resources I cannot get here.