The only thing that I am focusing on, other than the accuracy of measurements for bolt on dropped spindles is QUALITY. I wouldn't pass on the dropped spindles unless I have used them for a while on my bugs and have tested them to be sure that they perform flawlessly.
Narrowed beams dont effect the height instead they effect the caster angle of the front wheels. Once you go lowered, the caster angles on the front wheel changes and to keep it in control beams are narrowed down and shims are added between the beams and the bulk head.
Narrowed beams are narrowed at tubes length not the distance between the tubes. So on the narrowed beams, the torsion leaves are shortened too.
About adjustable beam, these are used to go lowered without adding dropped spindles/narrowed beam to a maximum of 3inches. Anything lowered than that would need dropped spindles.
There are mainly two types of beam adjusters, "AVIS" & "SWAY-A-WAY". Its a matter of choice which type adjusters you want to install. However, personally I would recommend "sway-a-way" adjusters if the lowering only is limited to adjusters as AVIS I dont believe would be appropriate for our road conditions.