I use an 1/8 oz. spinner bait a lot on a fast action spinning rod with medium power.

My question is what would the pro's or con's be if I went with a medium power and x-fast action rod ?

This is going to depend on several other factors to get the entire picture.  Rod manufacturer.  Length of rod.  Reel.  I dont know that you'll see a huge difference between the fast and xfast.  If you do see anything I think  you'll see slightly easier casting of the lighter lure.  Maybe.

B