I would just keep both. On bosses, I would switch to the block gear, but on minions I would use the critical gear. Truthfully, I faced Dimachaerus like 12 times. I was with a person with 50 block, I have 179. The boss hits me with his Natural attack, and lands a critical-no-block. My friend next to me, Dimachaerus does the same attack on him, criticals, he blocks. *Facepalm*. It's your choice but its timing that's everything.
if you are on celestia or higher I would chose block because the monsters have the ability to critical and block, so critical becomes less useful . I currently equip the darkflame bow ( or something like that) and it gives me ten critical and block. I am lvl 51 ice