Excellent and helpful comments, everyone.
I've chosen these four "representative" styles from looking at superhero artwork done in the 3D communities I'm involved with. And I have been keeping my personal opinion out of the thread so far.
ProphetSword, I've been doing my characters in the realistic style, but purchase of new characters has lead me to broaden my horizons.
hans, your observations are dead-on. Remember Image comics in the 90s? The characters, by which I mean the male characters, were so muscle-bound that even their guns had muscles. They would have called Arnold in his prime, a skinny little geek.
Dinonykos, you raise another good point in regards to the female characters. I have both the ability and the prediliction to make physically strong female characters as muscular as they would be if they were male.
marainein: I used DAZ Studio to do these renders. The characters are all "dialed-in" meaning someone else created the look and I just used it. My only "real" work was in making the costume fit, as it was designed for a very different base figure.
When I do my own characters, I use ZBrush for modelling to use in either Poser or DAZ Studio. I do a lot of my own textures and maps, which I generally make in Paint Shop Pro 8, but occassioinally use PhotoShop and GIMP, depending on the tools I need.