Well they're different styles of RDAs. Mephisto is the typical 3 post cloud blowing dripper which can take any build you want. Magma is a flavor machine that's simple to build on and holds a lot of juice.
Personally I adore the Magma. But that's just my style of vaping, I like flavor and the juice capacity over blowing clouds and using some crazy thick wire build. If you want to build a sub ohm build with thick wires and blow clouds, then you probably want the Mephisto. Although I'd highly suggest becoming more familiar with building/batts/safety before diving into sub ohm builds, since you say this is your first RDA.