They are two different things, Micro-set is used to place the decal first.. you paint it on the body then once the decal is down good you paint a little on top of the decal. It softens the decal and helps it settle down into the panel lines and such better. Also helps to eliminate silvering (tiny bubbles trapped under the decal)
Micro-sol on the other hand is the muscle of the two. Unruly decals, lots of contours on a spot where a decal goes is where this one comes into play. It's more aggressive and will REALLY soften a decal. When you first put it on it wrinkles the decal up and gets you really scared! Not to worry, by the time it dries it will suck itself down flat onto the model and be pulled into every nook and cranny that plastic has!
That said you don't ALWAYS need Micro sol. I've never had it happen but I have heard stories of it ruining decals etc... usually I only use it when I need to. The decals may be a little thick, they just don't wanna set themselves into the grooves etc... I always push the decal with micro set down with a damp paper towel first, and let it finish setting before making the decision to use the Micro sol...
They have directions on the bottles that are pretty self explanatory. The one part where it says "DONT TOUCH THE DECAL AFTER USING MICROSOL" Is a good one
I didn't listen the first time I tried them... you gotta wait for them to dry. (I got scared because it wrinkled up so much and was trying to dab it down like with the Microset)