There's a bug in 7.1.1.1 which crops up when you add multiple scenes to a YW5 project. The new beta 7.1.1.4 fixes this, and I'll promote it to Release shortly.
I also fixed the issue of dragging and dropping TXT and RTF files onto the scene window to import them directly into the selected chapter. It was a Winforms issue, not a bug, and I've managed to code a workaround.
To my knowledge, 7.1.1.4 still has a bug where pressing Enter on a selected scene might open something different - this is related to a 'focused vs selected' issue. And holding down the 'up or down arrow' on the scene list can cause the selection to go crazy. (Solution is to just press and release.)
Finally, there's an issue in the RTF importer where the character '𐍈' doesn't load properly. I have code to load double-byte languages (charset 128 and a couple of others) but Microsoft saves this particular as charset 1 - which I didn't think was a double-byte set - and then encodes the character with double-byte format. Esoteric, perhaps, but it's something I'm working to fix.
But to answer the question, I believe version 7 is the best one to use if you have the latest Dotnet 4.8 runtimes installed (Windows 7 and up.)
Version 6.6.1.4 beta is identical in every way to version 7, except the rich text box doesn't handle unicode as well (Winforms issue, not a bug.) It does work on Windows Vista, which is the only reason I can think of for using it. (Vista, and perhaps under Wine on Linux.)
No reason I can see for keeping yWriter 5, unless you're running Windows XP.