Completely separate market. Twilio is targeted at business applications--sending and receiving SMS via an API. It's not meant so you can text your friend, it's meant so you can enable SMS in your app. The pricing has fallen quite a bit, it was 3 cents per message when SMS functionality was introduced in Feb 2010.

HeyWire doesn't seem to offer an SMS API. If they did that for free, I'm sure they would be out of business in no time flat.