90% of startups will eagerly give you a free tshirt just for asking. They love the free marketing. No need to do stuff like submit builds, etc. I have gotten free stuff from: - IFTTT (socks) - MixPanel - CloudFlare - Elastic - infochimp - pingdom - moz - rethinkdb
Do people really want to advertise tech companies by using their merchandise? You couldn't pay me enough to even entertain the thought. Lemme just pull up that poorly sitting abhorrent short sleeved Microsoft Office business shirt real quick and we are off.
Snarkiness aside, I'd wear Microsoft gear.