I just had the same dilemma, I opted for decorative gravel with a wooden border to hold it all in. I've used bark previously and a lot of it got blown away with the Caithness winds, the weeds can also root in it and in my opinion it's a bit higher maintenance than gravel although probably cheaper.
I just used two ton of gravel costing £30 per ton, well worth the money as I'm really pleased with the result. I used the flat wooden borders from Homebase @ £2.99 for a six foot piece, I cut bamboo down to 10" sticks hammered into the ground to support the wooden border, the gravel supports it from the other side.
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