Mario Super Sluggers is, in some ways, an improvement; but for the most part it lacks in major areas.
My favorite part of the game is that ther are pleanty of different characters. There are many different combinations for you to set up. Some are obviously better than others, but besides that, it's a great way to get you sucked into the game.
Some of the weaker areas of this game include the shallow gameplay. It's very repetitive. Also, the game jumps from insanly easy, to glaringly difficult, and back then forth.
One last thing which makes this game a little better is that there are many different minigames. So, all in all, I would say rent it first, unless you're positive about this game. In some ways it's better, and some ways worse. In the end, It's up to you.