Personally I like air bags because they are adjustable. I am not a fan of just changing the blocks on the springs.
If you do the bags you will need the compressor to go with it. When I got mine I went cheap and didn't get one. Big mistake. We visited 26 places in our first 3 month trip and all the air up and down got old. Not fun to ride around with 60 pounds in the rear when crusing around sight seeing.