Just to avoid some confusion:
Hams have traditionally talked about "RG-8", but that military designator has been
discontinued and replaced by RG-213. While there are reputable companies still
making "RG-8 type" cables, you can also find junk, as there is no longer an
enforceable standard for it. There was also an RG-8A version, but most hams
ignored the last letter. RG-213 (or a reputable RG-8 type) is a good choice for
low loss if you don't have to snake it around corners or in through a window.
RG-8X (or what Radio Shack called "RG-8M Minifoam") is a smaller diameter cable
(about the size of RG-59) that is a good compromise between flexibility and loss,
due to the foam dielectric. (Authentic RG-213 has a solid polyethylene inner
insulation, though there are some "RG-8 type" foam cables around.) Foam cables
have lower losses for the same diameter, but are more prone to damage due to
compression (being stepped on, tie wraps pulled too tight, sharp bends heating
up in the sun, etc.) While losses are slightly higher than RG-213, it is easier to
route through holes in the wall or other tight spots where flexibility is useful.
One source of confusion of course, is the habit of ignoring the last letter, in which
case RG-8X becomes RG-8, which has a different diameter and uses different
connectors. So generally I talk about RG-213 to avoid that confusion.
You can use VK1OD's handy transmission line loss calculator to compare cables:http://www.vk1od.net/calc/tl/tllc.php
For example, for 25' of matched coax on 445MHz, RG-8 or RG-213 is 1.3dB, or
about 1/4 of your power is lost in the coax. Using RG-8x the loss is 1.8dB, so
1/3 of your power is lost. If you are running 10 watts, that is the difference
between 7.5 watts and 6.7 watts, which you probably won't notice in practice.
With RG-58 (which is even thinner and more flexible) you lose 50%, so you'd
have 5 watts radiated. In most cases, that probably won't be significant in
operation, either, and the mechanical properties of the cable may play a
bigger factor in your choice.
Sometimes the differences in losses ARE important, but if you a choosing a
J-pole and only need 25' of coax, they probably aren't.