I have built both. The Sig kit is far superior in my opinion. There is more cutting of parts in the Sig kit. The thicker balsa parts (3/8" +) are printed, not die or laser cut. The Balsa USA is all laminated 1/8"material (lite ply and balsa).
The die cutting is poor and laminating is a pain. The Sig is a 5/16 frame, Balsa USA 1/4". The Sig fram is much stronger.