I spent many months learning how to 'restore' old photos. Both from courses and from my betters. I don't have much experience in the discussion between 300 vs 600 dpi. I would only say that - for smaller photos - 600 dpi - gives gives restorers the best chance at restoration. Both in terms of clarity (eyes) and in terms of repairing damage. So if your photos are big and sharp enough and are pretty much undamaged, 300 dpi is fine. The smaller the photo and the more damage to it, the greater the argument for scanning at 600 dpi.