Basically both give same boosts but different numbers.

Guenivere aa gives 50% boost to dmg and armor, and very little speed. The aa lasts for 8sec with a 36sec cd. You have to be close to pet during those 8sec to receive buff, if you go far from pet buff doesn't apply. Stacks with elixirs.

Mephisto aa gives good boost to dmg, speed and buffs all stats. I dont know exact numbers of boost but i was receiving around +60 str dex & int. The aa last bit shorter than guenivere but also have much shorter cd and bigger aoe. Once you click aa and receive buff you can go far from pet np, buff still works. I didn't see dmg boost stacking with 30%dmg lix tho