All these factors are very important in the choice of an antenna, and i'll come up with a quick example: a ground mounted L/4 vertical above salt water (let's say, on the beach of a small oceanic island) for the 40m band will put out a DX signal similar to an horizontal dipole mounted at about 30m high (horizontal antennas need height to obtain better gain at lower takeoff angles), and the dipole will need to be rotated towards the direction you want to work while the vertical is omnidirectional. If you look at the costs of building a 30m tall tower, adding a rotator to it and the mechanical challenge of making it stable in saltwater-impregnated sand versus the costs of a few wires and a 20m tall lightweight fiberglass pole, the choice is a no-brainer. However, move the same setup above a poor, dry ground (let's say, Sahara desert) and the horizontal dipole will be a much better antenna - and while you have a tower and a rotator you might as well make the dipole a 3 or 4 element Yagi and get an ever better signal.
What i tried to get is a versatile 20m/15m/10m antenna that offers good gain and is easy to build, install and work with. I have used full wave loop antennas before and liked them alot, especially for the rewarding performance/complexity ratio. You just need a wire, cut it at about 105% of desired wavelength, spread it as you can and off you go. When properly installed (vertically, as spread as possible), a closed loop antenna has some advantages over a dipole:
- slightly higher gain (about 2dB)
- less noisy
- less sensitive to height
- less influenced by the surrounding objects and enviroment