i would say master depending on your skill and what youre trying to skillup on. this is unrelated in a skillup sense but still relevant. i tried using master shield in campaign soloing figuring that more blocks = less damage + more tp. but i found that i had a much easier time surviving with my koenig (~220 skill)
so i think masters is better until you get to a point where the things you have to fight can actually kill you, then use a rank 3
I had the same problem as you. I just Level Synced in the Mire until my shield was at a respectable level. Got PLD to 75. Tried to solo for skill but got fed up of it, so I just went to Dynamis, popped Reprisal, fought Monk mobs and poped sentinel when they use 100fists. Works wonderfully.