They are quite different, actually. First off, M80 is DAT (diminishing abrasives) and M205 is SMAT (non-diminishing abrasives) which means that in all reality M205 is capable of more cut/correction for each individual section pass.
Another difference is that M80 has a lot of polishing oils in it and as such is well suited to single stage paints (without a clearcoat) since it will feed the paint while polishing it. M80 also has a fresh-paint safe sealant in it making it an ideal choice for body shops and any panels that were refinished less than one month prior to its use.
For a cured base/clear paint system, you're probably going to be happier with M205 when it comes to general use purposes.