You could do either I suppose. I must admit my photo back up disks are all multisession data disks. I burn them as data and keep adding until the disk is full then start another one. I also keep 2 copies and check them both every few months and if one is faulty I would just burn another copy off the good one.
I'll be interested to see what someone says about the photo/video cd as I've never used it. The data disks I burn play on stand alone dvd players linked up to TVs as well so you can bore your neighbours/relatives just by popping along with the cd.