I have owned 3 Road Glides and although I have not owned a Batwing model, I have rode them. I think the difference between the bikes is negligible, same frame, same motor, same controls.
I think biglew has his points but I have not found there to be much difference in the wind buffeting, course I changed my seat, windshield and handlebars on all 3 of my RG's, maybe that's why I didn't see a difference.
I think the BIG difference between the bikes is the seat position. The FLH bikes seem to push me forward towards the fairing more, I think Harley designed it this way to make more room for the passenger. I think the RG was fashioned for the Motor Cops at first and then evolved into what it is today. The RG, to me is more of a 1 up touring bike than the Electra Glide.
Anyway rental is a great idea, make sure you take your passenger with ya too!