Based on my experiences with t-shirts - and this goes for just about all merchandise - is to not make it comic-specific. Sure, the hardcore fans might want a t-shirt with a character of yours, but what about the people who have never read your comic? Choose a design that appeals to both fans and n00bs alike.
For example, I did a run of t-shirts with my comics (then) logo and a tagline, and nobody except a handful of readers cared. However, I did a line of t-shirts with a line from my comic that said "If you give me cookies I'll believe anything" and a giant cookie on them, and people went nuts over them (in fact, I've done a whole line of cookie-related merch that sells well).
Bottom line is keep it as universal as possible and you'll get more sales.