The only difference I've spotted is that you can edit the font in open captions but not in open subtitles. But i'm sure there's another point to it.
This bothered me too. From what I see, "Open subtitling" is a special EBU standard, and is subject to the restrictions of captioning spec (probably Teletext, but I'm no PAL expert). Therefore there's no raster resolution, no fonts, colors etc., just a "grid size" for the number of (unformatted) characters.
Open captions are intended to be burned into the video, but offer the handy benefit of import and export.
Looks like both types can export to STL and XML. But "Open subtitling" can also export to EBU N19 format.