Given wood prices and limitations (which have improved, I admit), if the soils will permit it, I'd stake it down with either rebar or Tposts, since you don't plan to move it and it will be quite long. The longer your "run", the more wood framing would have to resist warping in the middle. You don't have those issues with frequent stakes or tposts driven into the ground. 12' or shorter, I'd probably stick with wood, thinking I might move it later. Beyond 12', either you would be driving stakes to stabalize the wood frame and hold it down anyways, or the stresses would give you something closer to a lozenge shape in the middle, rather than a nice parallel sided rectangle. May as well skip the wood, and go straight to stakes. (again, soils permitting - loose sand isn't great for resisting lateral forces, you need more stakes [or sand stakes], and if its too rocky to drive posts...)