I'm kind of confused, since you posted this thread at the beginning singers board...
In general the one would be both utaite and YouTube singer, since that's how these two nouns are defined. (What you're referred to as probably also depends on the fandom. People on Nico Nico Douga would be probably referring to you as utaite and people on YouTube as YouTube singer/youtaite.)
If you're referring to the wiki. Then probably the site on which the singer is much more known at. However if someone is equally recognized on both sites we refer to them as both. Such as SquaDus.
The borders are becoming more and more indistinct. As it gets more and mroe common for utaite to also have a YouTube account and vice versa.