I think just enough gravel to *barely* cover the bottom is a nice compromise.
I recently found out that gravel isn't really needed unless you have an undergravel filter. With a powerhead filter, I had 4" of gravel that wasn't doing anything but holding extra toxins in the tank.
As for holding the plants down, some of the better plants are weighted nicely. No need to weight them down, unless they're 'drifty' because of the filter current.
My new 20H has fine grain sand, for the corys and loach to play in. Sand, I am told, is easy to clean... the yuk kinda 'floats' on top of the sand, and gets drawn to the vaccumm without stirring the sand up.