I used both in some applications. The goldenrod is only something like 15 watts I think, which is pretty low (that's for the smaller one I believe). I think the larger one is something like 25W, but I am not sure at the moment. Cost per month you will have to calculate using the cost per kilowatt in your area. What is that, something like 76 cents a month at 7 cents/KWH? Someone correct me if I am off. Some silica I use is reuseable, others are not. I like the goldenrod because it keeps working as long as it is plugged in, but I need silica for those areas where I can't plug in, or both if I need the extra capacity.
For something like 8 cubes, a goldenrod is way more than enough. If you get the silica packs that can be baked and "recharged" so to speak, that's probably cheaper than buying the rod, but the rod will last indefinitely and be usefull if you get a larger compartment in the future, and won't have to be recharged no matter how many times you open the compartment and humidify the air inside. If it's convienient to plug in, I would get the rod - it will be more usefull down the line.