Andrew:
I feel certain that the 5888s contain more ink. I had several complaints about the 888s (and don't even get me started in the Hauser refills) running out of ink quickly.
The 5888 IS a bigger diameter (could just be the thickness of the metal) and I have had good longevity from the 5888.
This is JUST opinion, but the ceramic ball in the 5888 seems much better to me. The 5888 medium point really puts down the ink. The 888. M and F points, not so much.