In the situation that you are 13 and choosing between a 10 and a 15, always get the 15. The optimal will always be the one that is exactly your level, but when you are leveling 1-15 you are leveling so fast that a lvl 5 shield will become quickly outdated. If you have a lvl 5 shield, go ahead and use it, but if you're going to spend some money and buy a shield, then you're better off buying something a bit higher level, since it will last you longer. I wouldn't worry too much about it though since the content is really easy and doesn't require you to have great armor. Just focus on finding an affordable shield so you can start getting SP for the Sentinel class so you can start acquiring shield skills.