An excellent synopsis from Juergen with which I totally agree.the production time of FR66s was limited, so only a smaller volume compared to FR64s was produced. In conjunction with the very good performance the FR66s became a collectable, so the price level is very high.
The FR64s will not be on exact the same level, but very close by. Due to the higher volume and for not beeing a collectable, the price is very fair, but with a fast increasing trend.
There are two versions existing (as with the most FR and Ikeda arms) copper and silver wiring. Just decide to your personal preference. The difference of silver als copper wiring are well known and the sonic difference of the arms just follow the experience we have.
(my FR64s are copper and silver, so I can compare, no principle preference )
I've had three FR-64s arms.....two with copper wiring and my current one with the original silver leads inside....

I subsequently purchased an after-market B60 VTA Lifter which makes tuning life far easier

The FR-64S at a current price anywhere between $1,200-$2,000.....must rank as one of the great high-end bargains for analogue lovers.
It delivers the same level of performance as my $7,000 DV-507MkII (which I feel is worth its price)

Yes, the FR-66S is slightly more relaxed, slightly more refined, slightly more controlled, slightly more involving.........but is it worth the premium over the FR-64S?

Only if one simply MUST have the best.......