dang..start off with that line here? that's a hangin' offense..;)
They do the same thing, but Poser's been around for a few years, and has many more capabilities, which ones I'm not sure of, but I seem to remember people complaining of Posing.
Daz Studio is fairly new, and works well with Daz products (whoda thunk?..;), and has a tie-in to Bryce, so you can do awesome backgrounds, etc. It's also free.
If you're just getting into people pictures, and aren't sure if you're comitted, start with Daz, then upgrade to Poser when you're ready..;)

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)