Between the choices of sight and feel, I would definitely go with sight. I have not really experimented with the feel aspect though but would be interested in trying some of the things mentioned in earlier posts.
If you add sound to the mix, I would choose that over both. I find if I close my eyes when a girl is inflating them and I cannot see how big they are getting it adds a lot of excitement for me. Also if were both sitting up and her legs are wrapped around my waste I will sometimes kiss her neck or chest what ever feels better at the time and I can hear every breath she is pushing in to the balloon.
Drives me absolutely crazy.