Well, a t-shirt would be by rights an uncoated medium, so uncoated is probably the way to go. Check with the t-shirt printer which way they'd prefer. Technically, it's kind of moot. If it's being silkscreened, they just need to reference your colors as separate spot channels.
"Lucy, you got some 'splainin' to do!" - Ricky Ricardo
They have coated and uncoated swatches to represent what the same ink will look like on coated or uncoated stock. Think of how an ink will look on gloss paper, where it is not absorbed as readily, as opposed to the look it has on a bond, where it is absorbed and spreads.
And I like my Beefy-T's with a coated ink also!
People tell me "Have a Good One!' Hell, I already have a good one, I just need a BIGGER one! - George Carlin
All of your t-shirt printed inks will absorb similar to uncoated, but keep in mind it totally depends on the fabric and color of t-shirt. Sometimes a funky cotton blend will result in a lesser color. Also, if you're doing iron-on transfers, remember to wash them inside out and let them air dry to make the transfer last longer.