"Wearables," like this baseball cap, are by far the #1 promo product in the US. I think it's because it makes the wearer a walking billboard for your brand.
One reason I favor caps is they're more novel than shirts -- which I think makes the gift more memorable and a more effective marketing tool. (And having someone glance at your cap is usually more comfortable than them staring at your chest or stomach trying to read an ad.)
"Headwear" is one-size-fits-all, which is also a nice plus. You don't have to worry about a hat fitting -- especially this one with the adjustable strap. There's no embarrassment over giving someone a wrong size. And you aren't left with an unusable inventory of odd sizes.