What should I play first to improve my piano skills: classical or modern music?
Playing classical music and pop music use different skills and techniques. If you want to master classical music – you need to improve on your sight reading and be precise and accurate in all your playing – most listeners expect you to follow the music exactly as written. This process is lengthy. A masterpiece of any classical composer can take months to perfect. If you play pop/modern music, you have lots of rooms to improvise and make up your own version. You will have lots of enjoyment playing and arranging different songs. This process takes shorter time and is more fun. However, your technical skills may not improve since you are not tackling harder pieces. I think you should combine both types of music. Choose a classical piece that is within or slightly above your playing level to work on. Continue to have fun playing pop/modern music.