Both seem to be randomly chosen. The messages are definitely not affected by the shield skill training. A successfull shield defense will give shield skill marks both with the "block" and the "deflect" message. And after gaining the maximum shield marks from a specific monster, both messages still appear when successfully defending with the shield without giving any more shield skill marks.
gremlin - karmic lizard, dark orc, acid vortex, staring eye - dragon turtle, fairy dragon, ogre lord, minotaur emperor - pit viper, orc butcher - greater mimic, dark elven wizard, carrion crawler, dwarven child - imp - ochre jelly, dark elven archer, king cobra, goodwife - kobold shaman, magebane eye, floating eye, quickling king - giant centipede, hill orc sergeant, mimic hivemind, greater claw bug, brown bear, large gnoll - giant bee queen, giant ant warrior, shadow centipede, kobold