I think this has been discussed at least twice in the last week.
I wear polyester running shirts. They are light, breathable, and dry fast.
They don't hold up as long as cotton (snag easily), but cotton tees are miserably hot and I sweat like a pig in them.
My goal is to be as comfortable as possible while not looking like a slob.
I don't do a lot of hardcore landscaping, so it works for me. If I need to do something that requires "getting dirty" I wear something more appropriate for the task.