Either chapter would be fine, objectively speaking. DFOA works 3 of the Denton high schools' varsity and subvarsity and FWFO works the other school and, I guess, their SV. DFOA has a Denton area assigner for SV. As far as varsity games, there are some options in DFOA. First, you probably WILL get on the field -- especially if you're wiling to work multiple positions. Second, there is a possibility of finding a crew (already assembled) if someone moves, retires, gets hurt, etc. In previous years, there have been at least 3-5 guys on crews that for whatever reason move on prior to the season starting. Finally, though DFOA is going to a draft for all games this year, there will be games assigned by the assignment committee -- 4A down and Six man games.
I'd bet in Fort Worth you'd have similar game prospects. FWFO I believe has a satellite meeting location in or near Denton while DFOA has all their meetings at one site in North Dallas. There isn't a specific meeting requirement for DFOA though those working varsity games are generally expected to make the 3 or 4 August meetings. Others will have already taken place by the time you get there and there are only 2-3 meetings once the season starts.
Call both secretaries and see which one you like better.