I've had a tank with both, and while the lions are tidy eaters, eels often are not--and it depends what you feed. Best was shell-on raw shrimp, since they could be cut into bite sized pieces for the lion and had the bit of crunch eels prefer, but I tried getting a variety to them, so would feed larger pellets and frozen foods that stay in mostly one piece. Having something that will go after the chunks that get away is helpful. You still have to clean the glass and do regular water changes, of course, and adding a snail or brittle star means that you DO need to target feed them since they really won't get enough to thrive from scraps, but it does reduce the time you need to spend with your hands in the tank. That can be a benefit if you don't have someone to monitor the fish while you're working, as lions and eels can be aggressive and territorial. Trying to clean with a lionfish displaying at your hand isn't super fun.
