AFAICT (not being a serious expert on the formats) the only big thing is that +R/RW supports seamless sector overwriting while -R/RW doesn't (yet; if it becomes important I'm sure someone will come up with a way to make it happen, though by then it may be too late). What this means is that it's possible with +R/RW to overwrite a single sector without disturbing anything else; this would be useful for packet writing (i.e. floppy replacement) and on-disc video editing (i.e. videotape replacement).