I just uploaded two headshots; they're both very similar so I'd love input on which one is stronger or better.

And while I'm here, which photo should be my profile picture?

This one looks more natural. You look very uptight and uncomfortable in the other one. As far as which photo should be your profile image, I think the photo you currently have is the strongest photo in your port so i'd keep that if I were you.

Cheers.

https://www.modelmayhem.com/portfolio/pic/36994421 is better.
Also, imho, either your current avatar or https://www.modelmayhem.com/portfolio/pic/28014345 are stronger than either of recent headshots.

Neither of those photos (they are not head shots) help your portfolio in any way.

My choice for avatar and headshot