Cyclothymia is the mildest, I think Bipolar NOS would be a bit more severe than that. The spectrum does top out at Bipolar I, schizoaffective is kind of a mixed bag and schizophrenia is not a mood disorder.

As a BP II I'd have to say there is a significant difference between the types, most people with are pretty functional, typically unmedicated BP I's end up inpatient or in trouble with the law when they hit a manic episode. That's been my experience working in the field anyways.