The spf is most important relative to each person's burn time.

For example, I burn after about 10 minutes of direct midday sun exposure.

An spf 2 will let me be in the sun twice as long before burning: 20 minutes.

spf 15 gives me 2 1/2 hours

spf 30 gives me 5 hours

spf 50 gives me a bit more than 8 hours, which is probably longer than I'd be in real direct sunlight anyway.

For someone who doesn't burn for 20 minutes, an spf of 50 would just be over kill, and even an spf of 30 might be too much.

My cancer-man pushes spf 30 for everyone all the time. I'll go as low as 15, but no lower. (but I do hats and clothes with it if I'm just wearing spf 15)