- How does the build quality differ between the admittedly pretty basic Cherry model and, say, a Archiss/Leopold or a Filco one?
Generally its safe to say the Leopold/Filco is build more stable than the current generation Cherry G80 boards.
- Will typing on a Filco or Archiss/Leopold with MX blues feel very much different?
It will feel different, but not by much. Personally for MX Blue, I prefer PCB mounted switches (as in the G80), for Blacks I prefer plate mounted. Some people are the other way around. You just have to try and see which one you prefer personally.
- Will it sound different, when keys are usually very slightly bottomed-out?
Yes. Depending on how hard you bottom out, it will be much louder. With some models (Filco more so than Leopold) there may also be a metallic ringing cause by the metal plate. Varying on the board, its kinda hard to even notice that. Some people are not bothered by this at all.