Everybody calls them Solexes, but they are actually Mikunis made under
license from Solex designs. So when people say "Solex" and "Mikuni" they are
using them interchangeably.
You can like the Mikunis because they are original and sound cool, and you
can hate them because they are expensive. They work great above 3000 rpm,
but can be made to work well throughout the range.
People have put Webers on 2000s but there are usually a few throttle linkage
details to work out. It seems that most Webers' throttle shafts rotate the
opposite direction from the Mikunis. Also, there is less general knowledge
available about effective jettings with the Webers, whereas there are more
people who can help you dial in a set of Mikunis.
For anything up to (and usually including) a full-race motor, 44's are
plenty adequate. You'll typically only run a 34mm or 37mm venturi anyway (on
the street), so what does the extra bore size get you?
And I've never seen any other manifold for the 2000 unless it was