I want to buy a KTM... Which one should I buy?

You should firstly choose your priority between naked styling and Supersport styling. if you like faired bikes you should go for KTM RC 200. It mixes up the performance and efficiency part returning a reasonable amount of mileage whereas if you like street fighter looks, you can go for Duke 200 with the same specifications of the RC 200 but a totally different style. The 390 cc versions are far better at performance but lacks reasonable mileage for a user.