I've had both and sold both. I still have a Mystic "in the family" as my old man bought one after liking mine so much.
My thoughts after lots of seat/wrench time on both:
Mystic performs, handles, stops and does everything better than a /5.
It is not a good two up bike and leaves something to be desired in the comfort department for extended runs.
A later model such as this is USUALLY NOT in need of a long upgrades list after purchace. Watch out for missing tranny circlip and factory installed rear shock.
/5 is a MUCH better touring bike and generally more comfortable to ride. Drum brakes are an obvious short fall and depending on the bike you get there may be a longer list of repairs/upgrades that need to be done.
I traded my /5 for a 92 GSPD after 18 wonderful years of riding. Hated to see it go but my needs for a bike changed now that I'm in a very rural area.
Can't go wrong with either...If I were forced to choose again today I'd go with the Mystic with two changes, install some higher reach handle bars and a R100R subframe to accept a larger rear seat.