What size car is most used? It's really personal preference. The larger blocks will give you a quicker result, larger pieces & fewer blocks. I like to work with small pieces, so I would go for a smaller qube. I LOVE my accuquilt, but dies are expensive and if you haven't quilted enough to develop a block size preference, you might want to make a few more quilts to see which size you prefer, or at least have a specific project in mind where you can use your dies.
it's tempting to jump in with dies for your new cutter, but start off with those you need for your next project.