I remember the 7105t being an upgrade of the 7100t. Don't remember addition of hearing aid enabled as one of the features, but could be wrong. For what it's worth, according to zdnet, "The 7105t keeps many of the same features of its older sibling, such as integrated Bluetooth, a speakerphone, and quad-band world roaming, but it adds a higher-resolution color screen, larger Talk and End buttons, and contoured keys for easier typing."
ZDNet RIM BlackBerry 7105t (T-Mobile) Review & Comparison
Edit: Here's link to spec's on zdnet site.
ZDNet's RIM BlackBerry 7105t (T-Mobile) Specifications & Features
I remember getting the impression at the time that the 7105t would be better in sunlight than the 7100t. But then a buddy got one, and it didn't appear any better to me.