I have a Ref 5se and love it. A Ref 6 will cost you twice as much as a used Ref 5se. If you buy a used Ref 5se at a good price, you can always sell it later for pretty much what you paid for it. So you could have a Ref 5se now and upgrade later if you want to a Ref 6 when the used price comes down.
There are tweaks you can do to the Ref 5se to make it sound even better. Replace the power cord with a Shunyata HC power cord, not Venom. Replace the tube dampers with Herbie's tube dampers. Change the fuse to a Synergistic Research fuse.