I skipped v12 too and think v13 is a big upgrade over 11. The lighting and sharpness of the graphics stick out right away as something you will see, everything just looks better. The batting helmets in both sunlight and under the lights at a night game are great. The stick'em\pinetar on the helmets, as Sammy would say, looks so real. My favorite thing though is the baseball caps. First time the camera got close to Garza in my first game I looked at his cap and thought "wow, his hat actually looks like its made from cloth". Of course this is just visual stuff and while it doesn't really effect the game itself it does add a lot to the game.
That stuff is the stuff you see and notice first. The important stuff though is, as others have posted, the better physics that determine how realistic the game plays. IMHO it is a big upgrade over v11. I think if you buy the game you will be pleased.