X-Fujinon 50/1.6 vs 55/1.6

I have a modest collection of Fuji X cameras and X-Fujinon and X-Fujinar lenses. While I already have a 50/1.6 lens, I recently saw a 55/1.6 online. The 55 appears to be larger. Did it predate the 50? Is one considered better than the other?