Dellortos, IMO sound and look best.  Whether they actually perform any
better is not something that I can really comment on, I'm afraid.

Well, as Montreal owner I know the discussion quite well.
The "double" Spica (2 4-cylinder Spica's to feed the 8 cylinder engine) is
VERY good, however has a bad name because of (IMHO) lousy maintenance of the
car owners.
Unlike Dell Orto's / Webers / Solex, the Spica system NEVER gets
un-synchronized etc.
It's always perfect fuel/air mix.
The internals of the pump are absolutely good for more than the engine/car
life PROVIDED regular maintenance is done.
More specifically, the oil filter should be renewed every year/10000km
Being a mechanical pump, the Spica receives it's lubrication from the same
lubrication system as the engine. It's own oil filter has the tendency to
block quite easily resulting in no lubrication.
And without lubrication.......
Another "problem" that could occur with the Spica is that fuel leaks along
the pump elements into the sump of the pump resulting in a fuel poluted
lubrication oil.
This however is pure theoretical, as the internals hardly wear and if you
change the oil regularly, you'll always have good lubrication.

Monti owners tend to like to keep their cars original, that is another
reason NOT to go for this conversion.
Within the Monti community (www.alfamontreal.info) there are 2 or 3 owners
that have the conversion done, but all praise the performnce of the Spica
compared to the carbs.
An Austarlian guy has converted his Monti to a full electronic
ignistion/injection which ofcourse works perfectly as well.

Advantage of the 4 cylinders compared to the V8 Monti engine is that the
4/cyl´s can make use of readily available double horizontal carbs from the
scarp yard.
The Monti has the difficulty that the inlet is in the inner V, so too little
space for double horizontal carbs.

If your Spica is o.k., just keep giving it good maintenancne and enjoy!