I don't have a tri bike, so all my racing has been on my road bike fitted with aero bars. I would recommend that you buy them from a LBS and then have them fit you with the bars on the bike. Your riding position will change a little, and they can adjust your position on the bike to get the most comfortable riding position while in aero. I've got my bike set up such that I can ride aero comfortably, and on the hoods comfortably.
Definitely put some aeros on your road bike- you'll enjoy lots less resistance into the wind (although sometimes when I'm going a measly 10 mph into a headwind I wonder just how much aerodynamics is helping ).

Oh, and the head and neck thing will take some getting used to. Even now, when I haven't ridden aero for a while, my neck gets a little stiff. It's definitley something your neck muscles have to get used to!