What is the difference between Pulsar NS200 and Pulsar NS150?

It's the CC which makes difference.

Pulsar 200 NS and 150 NS belong to the same naked family of Pulsars. The 200 is a bigger version, in terms of performance and engine capacity. Body of both the bikes resemble each other because they have been built on the same frame and most of the parts have been used common on both the bikes. The 200 NS has an option of wider tyres, a rear disc brake, a bigger front disc brake and liquid cooling. The 150 NS will provide nearly 16 BHP of power from its 150 cc engine whereas the 200 NS has a 199.5 cc engine, producing 23.17 BHP. The 150 is expected to be around INR 75,000 whereas the 200 NS costs at INR 91,500. Choosing between both the bikes solely depend on your need and the way the bike is to be used. The 150 NS would be a good option for city riding and is no doubt comfortable at highways also, but the 6-speed gearbox of the 200 NS will prove it to be a much better bike for the highways as well for short sprints in the city.