What is the best covering for a redwood deck to keep the wood's character?
By all means, coat all six surfaces.

My favorite wood protectant is raw (NOT BOILED) linseed oil, but it's not for everybody. It takes a mule's age to harden, and it wants periodic recoating (exposed surfaces only) until it's fully built up. Nothing quite matches the beauty of a fifty-year-old well-maintained linseed-oil finish over natural wood, though.