Agreed - they seem similar (monocle can even use twisted's reactor). But then I have some personal issues with twisted. IMHO it's simply not well organised. Every time I wrote something, I had to browse through the long list of classes in the API documentation to find what I need (and wonder about how many kinds of reactors are actually needed, and why are they split across so many modules, and ...). The way the docs are written also makes it not very inviting for new users (some pages just seem to say "here, yield does magic").

I chose diesel over twisted many times, simply because of how it's organised, but then again sometimes I had to use twisted, since diesel didn't have some features.

Monocle looks nice to me. If it does the same things twisted offers, I'll choose monocle. Then again, that was a very subjective opinion.