Love them both, have them both. Not an easy choice. My take at it, representing only my humble opinion:
-When seen from afar, the II/IIIa looks tidier, more pruposeful. IMO, prettier. But up close, the details of the II/III shine, and every little curve becomes an Art Deco masterpiece. Finish is outsatnding on both, equal or better than any other camera. Again, like them both.
-I do think the II/III has an edge in reliablility: by design, the RF is almost imposible to get out of whack, and the shutter can't fade. You need good ribbons, thou. ITOH, my IIIa (bought new by my father in 1953) has been my most reliable camera, bar none.
-As others said, the prewar Biogon does not fit the II/IIIa. Plus, the II/III's RF has a longer base- so focussing can be a bit more accurate.
-The II/IIIa operation is much smoother. That is, of course, a personal observation, and your mileage may vary, but I find that the new winding/coking mechanism has a lower effort, not to mention that the II/III shutter requires that you actually tension a spring as you rotate the dial to change shutter speeds towards "high gear". But you already know that from your Kievs ;-)
My conclusion: I find the IIIa is a little faster, so I use it with wideangles for travel/street. I prefer the focussing of the II, and the deliberateness it inspires (in me), so I use it with the Sonnars as a portait and normal lens machine.