No grip automatically does anything. There are old guys that come out to the courts once a year with their wives who, by chance, use a western grip and poke at the ball like they were swatting flies.
Spin is determined, not by the grip, but by the path the racket head takes as you stroke through the ball. No matter what grip choice you decide on, you should try to learn to control all the various spins of the game and not be limited by your choice.
Having said that, western style grips make it easier to swing the racket up from low to high at a sharp angle that produces the kind of topspin that has become popular since Nadal.
It is really not as necessary to think about the backswing as it is the follow-through. However, to answer your question, with the western, when your racket is back (backswing) to it's farthest point, the racket face is almost parallel to the court. The eastern, when you take it fully back, the racket face is almost, but not quite, vertical. This is not something you should think about too much.
A western grip is useful for creating topspin with the most efficiency, keeping the ball in the court, for consistancy, creating angles, driving a ball at the feet of a net-rusher and so forth. You find it used mainly by back cout players that hit mainly topspin, though, of course, good players can do much more.
The eastern is a good choice for a person with an all-round game, who likes to create a wide variety of spins from slice to flat to topspin of varying degrees.
Good luck with your choices,
B
