Just a note from my experience - if you do end up doing gravel, make sure you get the smallest, smoothest gravel you can find, especially if you plan on Corys as tank mates. Sand is optimal, but not a requirement.
Second, if you do not plan on adding live plants, keep the gravel to only maybe 1/2" - 1" at absolute most. The substrate is really just for your viewing pleasure, and I had a bad experience in my tank where I had too deep of gravel and it made it impossible to keep it clean (even though I religiously vacuumed the heck out of it twice a week). It ended up causing bacterial infections and a subsequent Ich outbreak. When I pulled the gravel out to switch to sand, I was amazed at the loads of gunk that I never got, even though I vacuumed so much.
Just my $0.02