IMHO LR Classic is a major rewrite of LR, not an incremental release from LR 2015. LR Classic is very buggy and, if you have the option, I suggest that you wait out a few more releases of it before taking the plunge. Unless there's some really important new function in Classic that you need NOW you'll save yourself a lot of wasted time by avoiding it until all the major bugs have been squashed.
I really can't see that Lightroom Classic is anything other than Lightroom CC 2015 with a bit of tinkering. It certainly does not behave like a major rewrite, unless the major rewrite has made an even worse mess of it that version 6.