9913 has low loss, but unless you're using it at 144 MHz or higher frequency, and using at least 50 feet of it between your station and antenna(s), you won't be able to take advantage of that "lower loss," because the loss in RG8X, for 50', below 144 MHz, is so low that if it were completely eliminated you probably wouldn't note the difference.
9913 loss per 100' at 30 MHz: ~0.9 dB
RG8X loss per 100' at 30 MHz: ~1.0 dB
Not much difference. The difference starts to become obvious when longer lengths are used, or at higher frequencies. There's a large difference in loss at 1.2 GHz, for example.
The obvious disadvantages of using 9913 will become very apparent to you as you start to use it. It's stiff (inflexible), yet quite fragile due to its internal construction. It cannot be walked on, stepped on, kinked, or even tightly bent (in a small radius) without significant damage. 9913 was developed originally for cellular telephone sites, where it would be used only in straight lines going up the sides of poles, at frequencies near 900 MHz. For that application, it is ideal. For many amateur applications, especially temporary ones, or installations where the antenna must be moved or rotated, or the cable must be walked on because it's exposed to that, 9913 is not a wise choice.
73 de Steve, WB2WIK/6