Every once in awhile, I lose a post, just disappears. I did respond to this when it was newer. But I was looking at unanswered posts and found this still here, sorry.
Praying mantis are considered beneficial in the garden, because they eat a lot of things you don't want, including aphids, flies, leaf-eating beetles etc. They will also eat bees if they can catch them, but I'm guessing that doesn't happen too often. However, the large ones will also small reptiles, frogs, or pretty much anything that walks past them while they are standing still, camouflaged.
The little ones are just juvenile forms of the big one.