I recommend the 54, I don't notice much of a difference in size compared to 56 mm cubes, and it performs really well.
I have also heard it said(I forgot where) that it seems like Dayan designed the cube centered around the 54 mm, and sied it up later to make the 55 and 56.
I would suggest the 54, but the 55 is just as good(from what I have heard), but many say the 56 is too big.
Edit: Also the standard and maglev are virtually identical, other than price. I find that the standard is very fast anyway.