I don't really have astrong preference, but the main point is that whatever you decide to use, it has to be of really high quality. Poor/average quality of either type will fall apart fast and after only a few washings will look like shite, pardon my french.
Just make it look really nice and hold up well, and people will buy 'em.
If you did force me to pick one at gunpoint... then I'd say high quality embroidery is slightly > high quality silkscreen.