Yes, horizontal works better for DX, regardless of mode. Most FMers use vertical because mobiles are mostly vertical. I used to run SSB mobile years ago with a horizontal antenna, and the coverage was pretty amazing in mountainous central California. Those mountains almost seemed to disappear. Of course, there are exceptions. I just worked a guy in the Twin Cities on one of our local repeaters - he's a good 80+ miles away, using a vertical. But he's on a pretty good hill, and it's got to be one of those unusual situations. There are 'cracks' in the hills to our north, and vertically polarized signals do make it through there. But overall, horizontal works a lot better. Maybe we should all go circular. I've used circulars, and they are a great convenience.