The difference, if any, is negligible. I wound up doing a clean installation unintentionally. For some reason, when I first installed the ML update, it seemed to install fine, but it gave me some wake-from-sleep issues (the screen often wouldn't turn on). The Apple Store tried reinstalling the OS, and while it appeared to work at first, the issue cropped up again later that day. I finally fixed it by going into the restore partition, erasing the SSD, and reinstalling Mountain Lion clean. I migrated from my pre-ML backup drive and was good to go from there. After all the installations, I had the same amount of SSD space.