For wood, I'd go with old-growth Cypress, knowing that reedwood will get ugly after discoloration after some years being naturally weathered in the outside environment. Other than that, I've had no personal work done with it. And Trex is more expensive than Cypress, but building with it is also another valid option.
Just laying it out there, but why not use bricks or fieldstone?
That's what I have in my backyard. Although when a gap presents itself after laying down the bricks, there's more work involved in using a power cutter to measure & cut the stone to the desired width, for a snuggle fit.
I have made four 5X5 bed raised a foot (one layer of bricks below the grass level for greater stability), each with a drip system.