They'll constantly be scratching any 'sand' you put in there, out. You'll be adding sand all the time - that adds up to throwing money out the door. It'll be harder to clean, meaning you'll have to constantly cat-scoop the poop by bending or on your hands and knees. Getting all the old feeds, and treats and things that are not 'scoopable' will be a PITA too. It *will* eventually hold more 'junk' than you want it to and will begin to stink. Sand has its uses, but IMO not in a chicken run unless you just don't care about any of the above and don't mind the extra work it involves.
A concrete slab you can 'broom' to make it so it's not slick. Then when it comes time to clean, a hose is all you need. If you wish, cut a big 4" or 6" diameter PVC pipe the length of the run, wash everything into that and have it so that it 'drains into a 5 gallon bucket. Empty that bucket onto your garden/flower bed/etc. It also cools the concrete and in the summer the birds really enjoy it as it helps cool them also and you know that it's actually *clean*. You can even put the birds up in the coop temporarily and use a pushbroom and soap and water if you need to clean any more for any reason than plain water.