lets say that a 700 dlr folding bike made out of alloy is lighter than a 700 dlr folding bike made in steel ...
but the real difference in weight are the components
Dahon and Tern have an advabntage cause they buy larger quantities than all others combined in components, hence they have usually better components on any price range than the competitors... better components means lighter weight
in my limited experience ( :-) I see at least as many steel bikes break than I see alloy bikes with cracks ...
old 30 year old experiences in alloy frames like harsh ride or breakage don't count anymore...
steel rust ( from inside out usually ) alloy oxidates ( gets real fugly )
The Dahon and Tern specific Neos derailleur is very robust ... and shifts great, and is protected.... ( I sold like 3 replacement ones all last year ) Also we have a converter where you can switch to regular mounted derailleur if you so desire ... Only caveat which I don't like on the neos is that the limit screws are a pain to adjust, and the wheel doesn't want to come out as easy ( for the first couple times )
just find a bike you like .... rear carries out of alloy are not that heavy, nor are the sks fenders , if the bike doesn't already come with them
Best Thor