It's my understanding that the choke size that will work best with a street driven b20 car is a 32 or 34, to use any more you'd need an engine that breathes and revs very well indeed, and you'd still lose some low end power. Both the Weber 40 and 45 can be fitted with chokes that size, and with the same sized choke on I'm not sure if you'd ever notice the difference between the two. The difference is if you keep modifying you engine you can only run the chokes up to 36 on a 40, but up to 50 on a 45. So the 45's have room for future modifications. Plus I understand the 36 choke doesn't necesarilly work as well as others do on the 40 - there's not as much vaccuum created by the smaller 40-36 venturi.

I don't know anything about the differences between a Dellorto and a Weber - I think they are designed to be as similar as possible. I find it odd that there are so many near clones (Dellorto, Solex, HKS) of the DCOE - didn't they patent the thing?