In steam there is not advertising for your stream.
I mean - if you dont tell your viewers to go there they cant see that you are streaming. Yes your steam friends will be able to just right click on you in their friends list and use "Watch game" but since the friends list are limited I dont see is as any advantage.
Maybe a steam group can do the trick but again its IMO way harder to build up community there at 1st place.
I am pretty sure that steam streaming also lacks a lot of the viewer experience options compared to twitch.
In twitch all your followers receive an e-mail when you go online. There you can make your follower get to you by twitter or facebook and your stream actually have a link that you can post (something I dont remember having in steam streaming)
So in general Twitch is MUCH better than Steam streaming, especially if you are a streamer with big fan base.
If you are new but you have the time to create a FB and/or twitter accounts and promote yourself Twitch is still much better.
If you are doing it just for your friend or something then maybe there is no difference between both platforms.
