Really depends on what kind of chassis you're doing.
I mostly do brass tube/rod and piano wire types, along with some not too thick brass plate. Much prefer an iron, just because that's what I'm used to - also because it's easier to direct heat to specific joints. My iron is a variable JBC with top temp of 400° - not really powerful enough (35 or 40W I think), but because it has a wide tip it does the job.
If you're doing something like soldering together more modern spring steel chassis, then a torch would probably be a good idea. But you'd also need some kind of iron for electrical work.
And like the man says, what's really important is clean parts and good flux!