You can watch youtube either way they are offered as a app on the ATV3.
Personally I would use the ATV3 method so I would not have to deal with a long HDMI cable and being able to set where ever I want in the room and stream also not having to worry about a possible HDMI to ThunderPort handshake issue.
For 99.00 the ATV3 offers a lot of bang for the buck it will also mirror late model iPhone/iPad.
Nice to have when friends and family are over looking at photos/ home movies sent wirelessly from your late model Apple products with the ability for them to show their content also wirelessly from their Apple products.
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