Backpacks are definitely the easiest to move around with – and India requires a lot of moving around! Backpacks can be squeezed into train/bus luggage racks or under seats, and obviously, they go where you go. The size of your backpack depends on the bulkiness of your gear, but it’s a good idea to choose one with lots of zip compartments and pockets, so you can find stuff quickly.
Suitcases are less suitable (especially the four-wheeled spinners) for India’s rugged road surfaces and tight spaces, but they do make life easier if you’re staying in a place for a long period of time. I personally like this Samsonite wheeled duffle for the best of both worlds.