Totally different beasts. The Atma-sphere, given the right speaker, is a wonderful sounding amplifier. It is severly limited by the speaker's impedence. The sound is very transparent and fast. On some speakers, the sound can be characterized as whitish. On others, it can be quite good.
Unless your speaker has a high overall impedence, do not expect great bass slam.
The VTL is a powerhouse amplifier. It is designed to drive the most demanding speaker with the most demanding speaker load. The sound has a bit more body than the Atma-sphere and some would characterize it as a warmer sound. Not quite the clarity of an OTL nor the speed. Amazing control for most speakers.
Now, with such generalities, it would actually help to know what models you are considering as well as which speaker you will be using.
