Seasonic Focus Gold 550w...Superior platform. Seasonic actually makes and designs their own power supplies. The Whisper series is decent made by CWT. Bitfenix just puts their label on the PSU's. They don't design or manufacturer them.
They are both quite good. I would go seasonic personally, mainly just because if given the option I would take a seasonic built PSU over a CWT PSU (CWT is the OEM for the bit fenix whisper) cant really give you a good reason as to why. Mainly because everything seasonic makes is quite good, CWT makes good stuff and some less than good stuff. A review of each below.