Goat. I'll go against the tide - why on earth are you considering a Twin Otter for a 200 nm to 300 nm flight sector? As you also mention the Metro and B1900, I assume airstrip dimensions are not a consideration?
The Twin Otter is a great machine - for the task it was designed for!!!
A Series 300 over a 250 nm sector, return fuel, 30 minutes plus 10% and the first leg payload is 720 kg.
One way fuel, 30 minutes plus 10% and the payload increases to 1,290 kg.
A Cessna 208B Caravan will beat a Twin Otter payload over the same sector.