I agree I think they're kind of equal. However, I haven't ever tried the oval mouth but I do have my horses in French links and have great results. Especially my "greenie" mare. My friend rides all her horses in oval mouth snaffles and her horses all ride great in them.
Like kitten_val said they are both the same in terms of severity on the mouth, but I read/heard somewhere that the oval links are nicer on the mouths as they have a smoother surface, rather than the narrow link of the french snaffle pressing into the tongue.
The oval links are less severe. The flat link of the french link can rotate and put pressure on the tounge when the bit is in use. Because the oval link has no flat surfaces it lies across the tounge without having any of the rotational movement. I love the oval link bits and use them a lot.
And as always, when choosing a bit, just because it's not severe, doesn't mean the horse will like it. I like new balance shoes, you might like nike or reebok--horses mouths are not the same. I have a very sensitive gelding, and it took me YEARS to figure out which bit he liked, because I never tried it because it is labeled 'severe'. It's a waterford bit--and it's the ONLY bit he's happy in (loves going long and low and creating contact with my hands).