I use gravel because of the aesthetics of the type I have.
I loved the play sand I was using, but I didn't like when the bichirs would go crazy and would have sand grains stuck to them. I would see them scratch a lot and I got the feeling it may irritate them.
Also, even with my intakes at half depth, the sand still got in the filters.
I also tried bare bottom, but when it's cold all the hairs on the back end would stand up... aaand I'm taking that joke too far huh? lol
Sometimes I feel like an jerk for risking it by using gravel, especially since my ornates are the type of pigs that might eat the substrate accidentally, so I make sure to keep a sharper eye on those two youngins at feeding time.
Sand will always be best, especially if you can find that perfect grain size.