The ideal sun protection factor for you depends mostly on your skin type . This is determined in part by how much pigments are present in your skin. The paler your skin, the more often you'll have to apply sunscreen.
Unfortunately SPF is not as simple as it seems in terms of mathematics. For instance, don't think that a cream with SPF 30 is twice as good or works twice as long as an SPF 15. Also, applying a double layer of SPF 15 doesn't offer the same protection as a single layer of SPF 30 .
The difference in sun protection between SPF 30 and SPF 50 is not that big. SPF 30 blocks 96.6% of radiation while SPF 50 blocks around 98% of it. No sunscreen blocks 100% of UV radiation so SPF 100 is actually very misleading.