Aggressive clays are used to clean up more severe and rough paint contamination or overspray problems. Most of the common consumer clays are going to be the mild versions, which are fine for most applications. Unless you're working with a very bad finish, the milder clay will do the trick.
I just used the aggressive on my wifes daily driver that was loaded with dried up sap. It removed alot of it but not all. First time I used clay and will definately use it again. Megs was the only kind I could find.
The blue "mild" professional Meg's clay is a bit more aggressive than their white "consumer" clay. The red "aggressive" professional clay is the most aggressive. I'd stick with blue or white.
The abrasives in the Meg’s red clay (C2100) are aggressive enough to cause marring on some finishes. It’s intended to be used when you know you’ll be buffing afterwards.
I’ve used the white clay a lot and it’s always worked fine for me.
Other advantage3 with the red or blue over white is that it comes in an air-tight screw-top container. The consumer grade white comes with a little holder, but it's not air-tight -- you still have to do the zip-loc method . . . .