I would say go with what you're the most comfortable doing. Also, do you have any idea which position puts your body into a more aerodynamic position on the bike? On your next ride either have your riding partner check your position while you try different positions or look at your shadow to see which position make you more aero. Another consideration is the distance of the TT. If it's short, say less than 10 miles, I don't think it would really make much of a difference which position you used. In that case I'd go with the position that I felt the most comortable with.