Siri on the watch is the same as Siri on the phone, the only real difference is Siri on the watch will often tell you that you have to continue the question on the phone (e.g. if the answer requires searching the web or something of that nature).

Also, I haven't tested with iOS 11, but Siri on iOS 10 still recognizes songs just fine. I haven't tried recognizing a song with my watch though (and the question there is if the microphone on the watch is good enough to pick up the song well enough to identify).