The CP is a different form factor, only fits certain Antec cases
Other than that it is the better psu
 
Review on the CP850: http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=142

If you haveone of the cases that this PSU wil fit into, then it is a great option. If not, then the Corsair is your only option. If you get the GS700, then get the new version: GS700 (75-001382). The model number of the old version is the CMPSU-700G. You don't need the xtra power of either of those PSUs unless you're going to SLI the GPU in you sig.
 
The other issue is that the GS700 (old and new versions) only have two 6+2 pin PCIe power connectors and you will need four 6-pin PCIe power connectors too SLI your GTX 560 ti; you would have to use adapters to get enough PCIe connectors. The CP850 comes with enough cables. to SLI you GPU without needing adapters. If you're in the US, then the Corsair TX750W is a great deal right now for $84 after discount and Mail-in Rebate: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139021