The only fault with the K21 is that it won't spin with most crews without tail weights. Otherwise it flies well, it's easy to handle on the ground, it's robust, and at 12,000 hours it can be relifed to get another 6,000. You can't buy a new Grob these days an as you say the PW6 & Perkoz don't have the track record at present.
I have had a flight in a Perkoz, it flies beautifully but in my view had plenty of ergonomic issues. People who looked at it while it was at our club thought it looked fragile in key areas, and it wasn't as easy to handle on the ground. OTOH with the 20m wingtips it has an excellent best L/D.