Sand or Gravel?

I'm probably only a few days away from bringing home my 4' freshwater
aquarium, which I aspire to eventually have fairly densely planted. The
filter is similar to an external filter (except that it's incorporated into
the hood) with the filtered water returning at the top of the tank. The
hood includes 3 x 4' tubes.

The question: What do I buy for the bottom of the tank, sand or gravel?

I like the idea of black sand with perhaps a 25% gravel partitioned area in
the center. But I wouldn't really have a clue, so will appreciate any
advice on what will work best for the plants. A very high priority is to
keep it simple, so there will be no C02 or skimmers or any of that fancy
clobber--just water, driftwood, lights, plants and fish.