There are a few advantages to monosilicic acid over potassium silicate, the first being that it doesn't add any potassium which you may not need. It also is immediately available to the plant, where potassium silicate needs to be broken down by microbes first. It also doesn't precipitate out other nutrients when you add it.
I don't think microbes come into play with Potassium Silicate. Potassium Silicate has been used for years in hydroponic setups with no microbes, and can be fed as a foliar spray.
Source Document for info below.
"Potassium silicate is the potassium salt of silicic acid, and, in formulation, is readily absorbed by the plant. Silicon comprises 32% of the Earth’s crust, and silicic acid salts (silicates) are the most common form of silicon. Consequently exposure to silicates is widespread in activities involving contact with soil and natural water. The other part of the chemical, potassium is a required element for survival of both plants and animals, as is silicon.
Potassium silicate is an active ingredient to be used as a fungicide, insecticide and miticide.
Potassium silicate will be used as a broad spectrum, preventative fungicide with optimum control obtained when used under a scheduled preventative spray program.
Potassium silicate also provides suppression of mites, whiteflies, and other insects. It is approved for use on agricultural crops, fruits, nuts, vines, turf and ornamentals"
That said, MSA is 100% bioavailable to the plant, whereas Potassium Silicate products are only 46.7%. Gram for gram, ml for ml, MSA may be more effective, but it also costs more in most cases. The biggest benefit I see in MSA is that it won't jack the hell out of your pH if you're running hydro or coco setups. Potassium Silicate aqueous solutions have a pH over 11 and it takes a lot of pH down to counter it. In soil or buffered mediums, I'd weigh the cost of the two options and purchase accordingly. Also to
@InTheShed 's point, if you have a base Nute program that's already K heavy, you don't have to worry about the additional Potassium with MSA. Personally I use Potassium Silicate, but I've accounted for the lower bioavailability and the extra K in my elemental targets. I'm also in buffered soil-less where pH of my nutrient solution is not a concern. I'm a cheap ass and like the lower cost of Potassium Silicate.