Which RAM is better?

Solution
Much better off with the GSkill, the 2x4GB will run dual channel which provides up to a 10-15% increase, the GSkill also are known for OC headroom so stronger Mem ICS
The Crucial sticks have better latency (CAS 8 vs 9) and are low profile, thus posing less risk of getting in the way of aftermarket CPU coolers.

However realistically the two will perform almost identically. If you already bought the G.Skill ones I would just keep them unless you think the $10 difference in price is worth the hassle of a return/buying a new set.