The stock BeePre has quite a bit more technology built into it compared to the stock Smash. With both preamps fully upgraded, they have a similar level of technology, but the BeePre will sound better, perform better (lower output impedance, lower distortion, etc), and offers higher quality parts and more chassis space.
Having said that, the fully upgraded Smash is no slouch, it's considerably easier to build in its stock form, uses tubes that are far more affordable, and allows you to better hear the sonic changes that occur with the C4S upgrades and shunt regulated power supply.
-PB