Wearing slogan-less clothing is more low variance; probably no one will object, but likely none of your responses will be as positive as the maximum positive response from wearing a T-shirt with a slogan on it.

Depends what you're wearing instead. T-shirts with cute slogans on them are nowhere near the only way to make an impression with your clothes; in fact, I'd consider them a pretty cheap and lazy way to send a message as such things go.

You're probably right if I'm to take a plain T-shirt and Levi 501s as the implied alternative, though.