I'm another promoter of the double-jointed snaffles. My boy was in a single-joint snaffle forever without any major problems. But whenever I asked him to back, he would raise his head quite a bit. "Momma, this thing is jabbing me in the roof of the mouth when you apply pressure!!"
One attempt with a double-jointed (happened to be a french link) and no head raising at all!
I've tried both the French Link and Oval mouth with my boy, and haven't really noticed any difference between the two. Be sure to steer clear of Dr Bristols though! They can look sneakily similar to french links! But they're bad news. The center piece does not lie flat on the tongue, the sharp edge stabs into it!