Part of the decision depends whether you want to optimize performance
for 40m or 15m, rag chewing or DX, and on your soil characteristics and
available antenna supports, etc.
Generally I'd probably choose a horizontal loop resonant on 80m and
mounted at 90' or so among some trees. But if I wanted to work my
buddy 100 miles away I'd lower the loop to 30' and size it for 40m instead.
Generally a high horizontal wire will outperform a ground-mounted vertical
unless ground conditions are very good: I remember one ham in a logging
camp in Alaska who had a horizontal dipole up about 15 feet, and I kept
trying to talk him into trying a vertical because the camp was on a log
raft floating in salt water.
If you are considering a half-size G5RV, spend some time reading VK1OD's
analysis of feedline efficiency at http://www.vk1od.net/antenna/G5RV/index.htm
You will have to decide how much power you are willing to lose in your
feedline on which bands. One of the most efficient options is to feed it
with open wire line all the way from the antenna to the tuner in the shack,
and in that case there is little benefit in cutting it to any exact length (unless
you want the pattern aimed in a specific direction on a particular band.)
Any center-fed wire longer than about 40 feet or so can be pressed into
service on 40m, but the efficiency improves as it gets closer to a half wavelength.
The radiation pattern will be mostly broadside to the wire on 40m and 20m (unless
the wire is much longer), and will start breaking up into lobes on 10m (unless the
wire is less than about 45 feet long.)
So the "best" antenna depends on the preferred target location, the bands you
like the best, the characteristics of your site, how stealthy you need to be, the
effort you want to put into having a good signal, etc. If you are going to use a
G5RV variant, choose an efficient feedline option. If you are using a vertical,
use an adequate set of radials for it. With other sorts of antennas, consider
the feedline losses - just because the tuner can match it doesn't mean that you
might not be losing 75% of your power before it reaches the antenna.