Having flown both types, I agree with GG that I am not aware of any operator that flies the B1900 single pilot routinely. That includes one private operator of the type that I am familiar with.
If the operator has a solid SOP for multi-crew operations in the B1900 - then you will possibly get the best of both worlds (ie multi-crew experience AND fairly rapid progression to command, if you're good enough...). I know many B1900 pilots that have moved to jets as the next step.
The Dasher might have a longer queue for command in store for you (no problem if you want regional flying as your career, but may be a problem if you seek the majors), and multi-crew ops in the DHC-8-anything is no more challenging than in the B1900 - save for the very ligitimate addition of a F/A into the multi-crew mix - but I'm not sure that would be enough IMHO to tip the scales in favour of the Dash for multi-crew vs the B1900 for progression.
Good question, though - I reckon you'll get some good discussion on the topic.