Guranteed Stops usually Not Worth It.
I agree with Shadowninja - the 3 pips extra just cannot be worth the loss every time you trade. I trade frequently with IG with their ordinary stops and they've always been honoured even when they close the trade two minutes late and the market has moved over 10 pips (either way makes no difference).
The good thing about them being slow to action stops and limits is that you can always jump in before them and nick a few pips. Try that with CMC and they'll still action the stop after you've closed the trade and land you with a new trade you don't want!
As was said, Forex is 24 hours so gapping isn't a problem. I suspect if there was some very dramatic news or the White House gets nuked, a guaranteed stop might save you a hundred pips but, as traders, we play the odds and saving 3 pips a trade is odds in our favour...