I did a comparison chart between the Sidekick 3 and Blackberry 8100. It's available here: http://www.geocities.com/sk3vsbb8100/
I may update this soon to include other T-mobile phones.
But since you are deaf, you really don't need all the extra stuff which 8100 gives to you.
Even though I don't like the Sidekick 3, I think that will be the best choice for you. But if you still want the Blackberry, I will say choose the 8700 and get the unlimited text messaging that cost another $14.99 on top of $29.99 data plan.