From the raw specifications, I can tell you that the S Pen only has one button, and Surface pen (the one that we will use) has two buttons and an eraser on the other end.
The Samsung Galaxy Note 5 had 2048 levels of pressure sensitivity, and Eve V (I think) has 1024. That's half of it, but I can assure you, as a normal user you won't see the difference.
And let's not forget the form factor. S Pen is a tiny twig. I actually can't imagine myself writing with it effectively. Because it's too small to grip as a normal pen. The Surface pen is similar to a standard (hexagon-shaped) pencil, just shorter. Like a pencil that's half used up