The main difference between the Lexus IS 250 and 350 is power. The numbers just refer to their trim levels.
Trim levels differ by features. Lower trim levels are more bare bones, while higher-trim-level vehicles are upgraded with more bells and whistles.
For the Lexus IS, trim differences are most visible with regard to the the engine. The IS 250 is equipped with a 2.5L V6 engine capable of 203 horsepower. This is pretty powerful, but it’s nothing compared to the 350, which has a 3.5L V6 engine that can produce 311 horsepower. If you’re hoping for a more powerful vehicle, the 350 trim is definitely the one for you.