I keep sand in my BW tank but thats just because my violet likes to dig. Sand is easier to clean for me but just make sure you stir it up each time you vacuum.
The one thing I like about gravel more then sand is that gravel does not get into your filter and causes it to get very very loud.
I disagree with that scat/archer growth theory. I got a scat and my archer both around the same time, my archer has nearly doubled in size where as my scat has stayed practically the same in a years ammount of time.... maybe because the archer hogs all the food though.
What were you feeding them? Scats are generally plant eaters and mine will not touch the stuff my archer eats (oscar pellets, bugs, krill) where the scat prefers shrimp pellets, dried seaweed, and algae wafers. Archers and Scats both get big. Archers get 8 inches and scats get 12, so neither would be good for a 30 gallon tank. Reedfish might be a good choice, and I may get one for my 75. Crushed coral looks really nice, but you must rinse it for a very long time. I need to go pic up some more for my tank.