On one hand, I second the suggestion about using a wire antenna - it
will be much cheaper, and you get to learn a lot more about antenna
theory in the process.
However, I can also give you some information on the Hy-Gain antennas.
The 18V (if I recall correctly) is just a whip antenna with a big loading coil
at the base. You have to change the tap point manually on the coil when
you want to change bands - this is not always convenient. (For SWL, you
could just use the whip without the coil: it would work as well as an 18'
vertical wire.)
The 14AVQ series trap verticals provide automatic band switching, so you
don't have to venture outside in the rain. (You can get the same
performance using 4 wires - one for each band.)
General comments: either antenna will require a good set of ground radials
for optimum performance (though this is not as important if you are only
listening.) Trap verticals of various sorts are often available used for
considerably lower prices. The primary advantage of a commercial
vertical is that it is self-supporting. If you have trees or other supports
available, you can get equal or better results using home-made wire
antennas.
Just to get started, find a piece of wire about 20 to 60' long and throw
it over your roof or string it through the attic. Attach one end to the
radio. You will hear plenty of signals.
Good luck!
Dale WB6BYU