I have both SP harpsichords and use them for different purposes. The Ruckers is much more mellow, and the Mietke more articulate. The Mietke extended version has a 16' stop which is more useful than one would think. It is not unhistoric, Haas in Hamburg was making them during Bach's lifetime.I believe, (correct me if I'm wrong) that in the inventory of Bach's instruments he owned such an instrument.
I had the very good fortune of spending time with a copy of a 1740 Haas with 3 keyboards made by Goble and sons. The instrument was for a performance of the Poulenc concerto with Trevor Pinnock in Ottawa. It was subsequently used for the recording with the Boston Symphony of the same piece. I spent one lovely evening all alone playing through the Well-tempered clavier, and it was very convincing with the occasional use of the 16'. The first manual was the 16', a half-register of 2' (bass to middle c', and the 2nd manual would couple to it for the pleno. The second and third manuals were like the traditional 1st and 2nd elsewhere. As well, the 16' is used for the 20th century concerto repertoire (Poulenc, deFalla), Respighi orchestral pieces and other orchestral repertoire which requires an expensive, difficult to find (and musically unsatisfying) rental.
The Mietke harpsichord has been incorporated into a claviorganum by Augustine along with a small Dutch organ from Oosterwitjdwirjd. It has been very useful to me for continuo playing There was a thread about this in 2015 with the links to download the necessary elements. It is necessary to own the Mietke harpsichord but the organ is free as is the claviorganum file. I looked recently and the files are still available. Here is the thread from 2015 viewtopic.php?f=6&t=13513