@joannekwan I'm surprised you had lag on the surface pro or the cintiq - I've tried Clip Studio Paint on both and they were silky smooth.
I find that the apple pencil is better for smudging and shading since you can use the side of the pencil tip, which is really cool.
In my opinion, though, in terms of computer functionality, inking, the presence of an eraser on the back, and the glorious built-in kickstand I'll take a Surface any day. My recommendation would be a Surface Pro 3 or higher - the Surface Books are good too but a bit expensive and the undocking process takes a bit longer than just putting the kickstand down on the Pro.