I'd definitely pick Guardian over Achilles, and probably HH+MW19 before Achilles as well.
Achilles reduces damage taken by 16%, while Guardian blocks 42% of attacks (average) and gives other increased tolerances. If you already have the HP to survive attacks, that 42% block far outweighs a 16% damage reduction. For pallies, I can't see Achilles being incredibly important. While training, we already take virtually no damage and use very few HP pots.
If you're training with an HS/MW19 mule bishop, definitely go Guardian and HH. Imo, all three are more important than Achilles.