For webcam, I generally only have mine on when playing myself, not when doing spectator commentary. When I'm commentating/casting, I want the game to be the focus, and my commentary to be complementary. When I'm playing, however, I figure the more personal touch of the webcam makes sense since I'm showcasing my own play as I'm playing, and it also takes some pressure off speaking, since my facial expressions can do some of the talking for me.
For resolution, use 1080p or 1440p. 1080p is the highest most will care about, but 1440p has tended to get YouTube to better preserve quality when it does its processing, so that might be worth the HD space.