That's a big question actually ;-)
The main advantage of Terragen is its renderer. The way it combines fractal shading, atmospheric shading, cloud shading to produce a render suited to landscapes. If you want to render with Redshift you only thing you'll be able to export is the geometry (from Terragen), and then you'll be essentially starting from scratch in Redshift/Maya trying to emulate a Terrgen render, which would a major project in itself (by the time you've figured that one out, your Terragen render would be finished :-) )