What is the best recipe for Chocolate Mousse: with butter or cream?
The percentage of milk-fat determines the name.  It is different from region to region.  

In the US, heavy whipping Cream is above %36 fat.  You can see this chart for the naming convention and fat content.  For a chocolate mousse you're generally looking for cream with fat contents between %30 and %40.  Otherwise you can get a phase-inversion in your mousse emulsion (it break or collapses).


There are Chocolate mousse recipes using butter instead of cream.  It makes for a slightly different tasting chocolate mousse, but hey, it's chocolate mousse.  
You may be able to cheat using an emulsifier such as soy lecithin to keep the emulsion stable.  Gelatin does thicken things, but you're likely to lose the air bubbles in the mousse and get a denser mix.