I don't necessarily have a preference but more often than not I rock a polo. Mainly because most of my polos are very thin golf polos and they keep me cool in the summer. They are also handy to wear incase I have a spur of the moment urge to go golfing.
I'm not going to pick one, because I don't know what it's for and that is how I'd decide. I'll usually go with a T shirt, but if I have to look halfway nice, but don't need to actually dress up I'd get a polo shirt.
Wow this is just what I needed, something to figure out. I wish I could pick both. I wear both all the time, I choose by just picking which is next in the lineup. The bad thing is that I usually pick something that doesn't work out well, then later in the day or night I wish the other was what I picked.
T-shirts are very casual agreed... but for me there's no middle ground between them and a proper shirt. I don't know of anywhere that's too casual for a proper shirt but not casual enough for a t-shirt.