Twitch has had some massive technical problems this year with big streams lagging or being delayed, E3 being a prime example. Their video quality is also not great.
The youtube gaming UI is just absolute SHIT though, its basically a skin on top of youtube. You can actually find the live playlist right off youtube even. And like people have said they need to fix the DMCA shit or nobody is gonna switch.
It'll be interesting to see what happens, amazon laid off a bunch of people this week from their hardware and gaming divisions so its sort of unclear where exactly amazon wants to go with all this side-business stuff. They have apparently moved everyone to work on a f2p DOTA-clone PC game which they want to jump into, and twitch would be a good bed for that.