If you only like healing, Priest is the safer option (i.e. with the freedom to play Disc if Holy falls off a cliff). If I was only picking one character though, I’d probably pick Paladin for their great carry potential in PUGs and general flexibility if you want to play other roles.
Ultimately it’s a preference call. You have both characters at level 70, so the recommendation is pretty much whichever of the two you enjoy playing more in dungeons and raids.