..Regardless of length, you want to keep at least two hoses. That way when (not if) you have one spring a leak, you can quickly grab your other hose and use it if need be. Nothing worse than everyone being ready to leave and waiting on you to drain the tanks, and you have to go find, buy a sewer hose on the fly.
As to length: I agree, that its good to have both a 20' and a 10' hose (or two 20' hoses would be even better). You'll find that most places you go just one of the two will be enough, but ever so often, you run into the sites where they have the sewer connection in a place where 20' is not going to be enough. You don't want sewer hose limitations to dictate how, where you park your rig on a site. Nor do you want to have to move your rig just to empty the tanks.
..You didn't ask this, but I'm going to give you this advice, also, as to the type of hose to get: You never know, when a hose may spring a leak and you will find yourself having to find, buy a hose in an area you're not familar with. That being the case, its best to go with the standard, common hoses with standard bayonet-style connections that can be bought just about anywhere.
All those "fancy-shmancy" hoses that can be run over, survive a hurricane, and have cool little connectors are all fine and dandy when they work, but ALL hoses will eventually leak. Finding a replacement or fittings for one of those fancy hoses somewhere out in the boonies can be difficult.