So I am also doing this and have chosen to apply for deferment to my law school of choice. I have a couple of reasons for this:
1. I've spoken to people at the law schools I'm applying to and they've said that deferments for TFA are generally rubber-stamped, meaning I wouldn't have to choose between the two.
2. There's no guarantee I'll get a TFA position at all, let alone in the locations I want it, and I don't want to be left with no job for next year or with a job in a strange place when I could have just gone to law school.
For me, the problem is that some of my schools won't defer merit-based scholarship offers, so I'd be forgoing those (or at least the first two years of those) were I to take the TFA position. So if you're considering applying for merit-based aid, you should talk to the schools in question to see whether they either allow you to defer such aid or whether they allow you to reapply for it after your TFA term.
I hope this helps!