A few data points pertinent to those PSUs:

OCZ ModXStream 700W:
- Very unimpressive - by today's standards - 550W at 12V rails due to the unit being a...
- ...dated design. (it's a 2008 PSU, folks)
- Cheap^WHigh-value components made the unit struggle at higher loads. (HardOCP)
- Very few PCIe connectors (one 6 pin, one 8 pin) necessitate the use of adapters.

OCZ Fatal1ty 750W:
- Quad rail design means you need to pay close attention to how you wire things.
  Moreover, the rail distribution is wacky:
    12V1 (solid yellow wires): Main motherboard cable.
    12V2 (yellow/red wires): Half of the ATX12V/EPS12V connector (ummm... wtf?).
    12V3 (yellow/green wires): Half of the ATX12V/EPS12V connector, half of the modular connectors (two for video cards and two for SATA/peripheral).
    12V4 (yellow/black wires): EPS12V connector, half of the modular connectors (two for video cards and two for SATA/peripheral). (HardwareSecrets)
- Two hardwired CPU connectors might necessitate the use of an ATX12V->PCIe adapter.
- The unit burnt down under an overload condition instead of safely shutting down. (HardwareSecrets)


As long as the limitations of both units are understood, the rebated price tags make this opportunity a no-brainer.