...Anybody have any opinions about which is better?...
Depends what you are trying to use it for, how high it is, etc.
I've used long wires up to 600' or so, and a number of different
loops. When properly fed any of them can work reasonably well;
the main consideration is the radiation pattern.
As a wire antenna gets longer (beyond about 5/8 wavelength), the
pattern breaks up into multiple lobes and nulls, especially on the
higher bands. If you are happy to work in the available directions
and ignore those where you have deep nulls, they can be satisfactory.
Same with a horizontal loop: when the perimeter is 2 wavelengths or
more the pattern is a series of lobes and nulls in various directions:
I like to use an 80m loop for Field Day, but on 15m and 10m the lobes
are so sharp that a significant number of stations fall in the nulls
(even though the ones that are in the lobes are strong.) In this
case I have a strong signal to some stations, but not to others. If
I'm just looking for a few good contacts it works fine: if I'm trying
to work as many stations as possible, there are some in the nulls
that I can't reach.
Personally I've had best results with long wires that are less than
about 130' (though a 270' one got excellent reports from the
other side of the state on 75m due to the lobe placement.) Beyond
that the lobes tend to be too narrow to be generally useful on
20m and higher bands. The 600' long wire was a particular
disappointment on Field Day, possibly due to its orientation or height
above ground. Similarly with horizontal loops, a full wave on 80m
is pretty good on 20m and usable on 15m if I can align the lobes
in optimum directions. (Last year for Field Day I put up a 40m
doublet instead and worked more stations than I had on the big
So both loops and end-fed wires can be useful in some situations, but
the patterns may not be favorable on the higher bands, depending
on your operating preferences.