well buddy my walking cat loved to bury in the substrate. its ur choice. id say go wit substrate. Hard work is needed to keep a successful tank. btw my clarius batracus was murdered. both of them, by my Channa Micropletes. Tiger Snakehead.
So you recommend gravel, eh? Hmm...must think about it...
It's a pity that your cats were murdered... snakeheads are killers really... wish I could have one... but they are rare and expensive here... and the fact that they eat only live fish makes it very hard to keep them... in Mauritius we don't get feeder fish at LFS... unless we get them from the local rivers... and that's not an easy thing to do...