Assuming both have decent IVs and both would be used for defense, I think trying to get a ZH-DG Blissey is highest priority.
ZH-DG Blissey is the best defensive pokemon and moveset in the game. And it SUCKS to go up against it.
ZH-BS snorlax also sucks to go up against, but not as much as that Blissey.
I don't care about defense. But if OP wants to use a TM on a Snorlax or a Blissey, two mons that have absolutely no use on offense, clearly OP cares about defense.
And I'm answering with defense in mind as a result. ZH-DG Blissey is objectively the best defender and moveset in the game, even if defense means nothing now.
I agree with everyone else that until the game changes again requiring half decent defenders, you shouldn't use TMs on any of them...
Also, moves can change so Zh/DG could be bad with a single move change (Gen 3 probably?)...
No point in burning a TM at this point in the game for minor benefits...Berry defense will work a lot better as well than simply changing it's moveset.