According to your wants, Taric would be better. Both Rammus and Taric are very hard to kill, but Taric provides better support than Rammus and pushes better than Rammus in the early game. For what it's worth though, Rammus is better at defending against pushes.
As for ease of use, I'd say Taric is easier. His skills are very straightforward: heal whoever's low, stun whoever's farthest, shatter in the middle of everyone, turn on ult for teamfights. Rammus' Powerball has some odd hit detection, and using it cancels the use of some other skills (and vice-versa).
In the end, Rammus is the truest tank in the game with his Taunt and DBC. Taric is not as good at taking damage but is more versatile. Imo you should just wait until they're free and try before you buy.