Should I go for Adata SP600 or Samsung PM800?

It would have to be one heck of a good price to get a used ssd. The problem is someone may well have used most of the write cycles(the limiting factor to ssd's life time) and then just be selling a drive that may not last as long as one would hope.

This all depends on the user clearly, My "worst" ssd has 21 full drive writes(wear level) in 512 days so I have used less than 1% of its rated lifetime, but another user may have burned up over 2000 in the same time(the amount of program erase[more technically correct than write cycles] depends on the drive as well. Some have more or less it is all down to the flash used). My usage pattern is read heavy and not write heavy at all since all my files are actually on hard drives.

I would personally go with the larger model just because 64 gigabytes goes fast and if you can live within 64 gigabytes then 128 will leave you lots of room for the ssd to perform wear leveling and other self maintenance.