I'd say use smaller and fly 2x/3x daily. That is for 2 reasons:
1- you do not have competition at the moment but you may have competition tomorrow. You would be frustrated to fly at 35% LF when somebody with 50 seat props show up and start running 3x daily to routes where you fly 1x.
2- if there are N routes with 150 pax demand, then there are probably 2N routes with 50+ pax demand. You can squeeze your base tighter and make money from those thin routes with smaller planes. With larger plane you would never fly those routes.
F50, Dash 8-Q400, BAe ATP, ATR are all viable options for you. You can pick one of them based on your demand characteristics and availability on 2nd hand market.