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!
On younger horses, I use a hollow loose ring, just because they are very soft and light on the horses mouth. I have never noticed any pinching and it works nicely.
When I begin to start jumping more I will move them to a different bit. One of my favorites is the Myler Dee Ring with a french link. Myler is awesome because all their bits are arogonomicly made to fit your horses mouth and works very well. Their products are a little more expensive, but I believe they are worth it.