Glass bubbles are used to reduce the density of the resin, thickening it while also making it much easier to sand. Great for making epoxy fairing pastes. 

Fumed silica is a thixotropic additive, which is different to thickness. It changes the behaviour of the resin under shear stress, meaning that when it is just sitting it will be very thick and not sag, but when you are brushing it or moving it, it will be much thinner and still easy to move around. If you've ever worked with gelcoat, gelcoats are a thixotropic substance. As is honey.

You sometimes might want to combine the two to get the properties you want. Glass bubbles on their own will make a nice thick paste, but it won't be thixotropic and so might still sag if used in thick sections of a vertical. It also makes for a relatively brittle resin, thanks to its low density. While fumed silica is great for making pastes that won't sag, but because it is derived from quartz it also makes the substance extremely difficult to sand.