Its harder to siphon because you don't want to pull up all of the sand. You sorta have to keep the siphon a hair or two off the sand in order to lift the waste without sucking up all of the sand. With gravel, you just drop the siphon into the gravel and let it suck the gravel up.
Once you get the hang of it, siphoning sand is pretty easy. Plus with sand, it is easier to tell when it needs to be cleaned.
the sand is so light that your syphon will take the sand with it.
I have heard that sand is easyer to clean b/c all particle stay on top. There is no crevases for them to fal into. But you have to clean it more often to make it look good. I am currently setting up a new tank with sand and all my others have gravel.