Youtube(founded on 2005) used to be the place you should upload your video while Ustream(founded on 2006) was the best place to broadcast/upload your video live.

Youtube Live was implemented 2011 giving Youtube live streaming capabilities just like Ustream.

Youtube.com has always been more popular and always had a much larger user base than Ustream. Although Ustream was growing in popularity while service the live streaming niche, Youtube Live has since dominated live streaming.

Another reason youtube.com/live is more popular than Ustream, is because Youtube allows you to implement Google AdSense.

According to this video created in 2010, Youtube Live is slight faster, as a website, than Ustream.