Go for Duke 250 ....it has a bigger tank 15l vs the tiny 9 litres of cb300r. Also it has better pillon seat which you can use for touring. Also engine refinement is on par with cb300r. Better lcd display on Duke. More features on Duke.
I have been using gen3 250 for last 4months. To be frank, machine is awsome. But if someone is taking it would like to provide a suggestion. Just change the rider seat and pillion seat. Those preinstalled seats are track focused . So you wont be able to go for a long ride. I tried 350 km for single stretch. It was too hard for me because of the road condition. Its better to install a cushioned seat.