NowJS supports clientId's and sessions, which would be easier for supporting multi-user real-time app. I think the both are practically the same and if you have some delay with socket.io, I really don't think it will disappear with nowJs.
I've switched to NowJS, because it's easier to structure your application ( for me ). I also experience a delay, but I think it is normal and since everything is async you could manage it in your script.
EDIT 1:
I discovered the cause of the delay. It seems that when NowJS is initialized it executes a lot of code to evaluate your "now" constructor, and synchronize it with the methods added from your server/client. In socket.io you have no such delay and it is faster
Update :
Nowjs is no longer in active development ( there are no recent updates and there probably won't be by the authors ). You should definitely look for something with greater support.
https://github.com/Flotype/now/issues/208