I've seen pictures of both, and they look very, very similar, including the small venturis/velocity stacks they both use, and the breathers. It looks like they use slightly different size filters (both K&N), but that's about it. I suspect they both function exactly the same.
The Thunder kit is less expensive by about $50. I'll be installing the Thunder kit myself, and I'll post pics and our review as soon as it's done.
"Big Air Kit" - Baron's Custom and Patrick Racing's name for their versions of a high-flow intake system.
The function - regardless of brand - is to remove (in most cases) the restrictive stock airbox assembly, and replace it with an open element system (usually K&N) which flows significantly more air into the engine.