If you will be using the light primarily for studio, and shooting at wide apertures, the b400 will do fine. I almost always have my 800s at super low power. The b400 will still be handy as a fill or background light if you find you end up needing more power.
b800s are more versatile with modifiers (grids), larger softboxes, greater distance, and outdoor use- although for the latter a b1600 would probably be better suited.
Sony A850 | Sony Battery Grip | Carl Zeiss 24-70 Vario-Sonnar T f/2.8 | Sony 70-200G f/2.8 SSM | Lensbaby Composer Pro | Sony HVL-F58AM
Alienbees B800 (x3) | Alienbees Cyber Commander | CSR+ Receivers | Grids, Softbox, Gels, Manfrotto/Century/Avenger stands