iPhone/ iPad considerations
If you also want to edit the file on iPhone or iPad, then depending on how you are sharing the file between devices (if at all), it might be more convenient to use .txt. For instance, Dropbox’s editor handles .txt. To deal with .taskpaper files, export is required cf. Dropbox page on export. However, your setup might allow your favorite iOS/iPadOS editor easily to open the files. E.g., 1writer can be given access to Dropbox folders and open files with .taskpaper extension.
Multiple macs and sharing TaskPaper files
Using .taskpaper can be more convenient in a multi-mac situation – otherwise (depending on your setup) the other Mac (than the one on which a particular taskpaper file was created) might not open your " TaskPaper" files that end with .txt files.
One reason: when your Mac is configured to open a specific .txt file with TaskPaper rather than another app ( say, BBEdit), some information in that file’s extended file attributes is set: xattr: com.apple.LaunchServices.OpenWith, sets a custom app to open a file – The Eclectic Light Company .
Some cloud services, like Dropbox, drop xattrs; so, even if you’re set on one Mac to always open a particular .txt file with TaskPaper, if you sync the file with such a service, and open it on another Mac, macOS will choose the default for .txt files. Actually, it’s ‘worse’ than that. Even on the same Mac, xattr info may be lost.
However, if you use iCloud, this .txt issue isn’t a problem, since iCloud preserves xattrs. But see Which extended attributes does iCloud preserve, and which get stripped? – The Eclectic Light Company
Mac , iPhone/iPad
Also, if you want to quickly search for TaskPaper files on iOS or other devices, using the .taskpaper as the extension does the job. I find that being able to quickly find files, particularly TaskPaper files, is quite important.