Corsair RAM is overpriced in my opinion. Yes, their RAM is renowned as being top quality but you pay too much of a premium for that privilege.
Take their Dominator GT DDR3-2000 C8 modules for instance, they're £355. You can get the same speed RAM from just as good a manufacturer (albeit with slightly looser timings) for less than half the price of the DGT modules. CL1 is absolutely not worth £180. I know some of their other brands don't have extreme price differences like this, but I still think that the higher prices are unjustified for the benefits they offer.
With what you've mentioned above, I'd definitely go for the G.Skill Trident 2000MHz RAM. The Corsair XMS3 C7 RAM is C7 at 1600MHz. You'll be able to run the G.Skill RAM at C7 at 1600MHz also (lower speed so you can tighten the timings).
Another thing I though I'd mention is that I remember reading somewhere that 1600MHz @ C7 performed better on a X58 platform than 2000MHz @ C9. But I'll be testing that myself when I get my new build sorted.