Both are capable of handling a system with two Radeon HD 6870 graphics cards in 2-way CrossFireX mode.
There's a 3 Year warranty on the Gaming Series GS power supply units and a 5 Year warranty on the Enthusiast Series TX power supply units. So it seems Corsair is more confident in the reliability of their Enthusiast Series TX.
The CMPSU-650TXV2 is made by Seasonic.
The CMPSU-700G is made by CWT.
Seasonic is the preferred OEM.