I rarely buy under 45. I am pale and pasty and burn easily, once the burn fades, I'm covered in freckles. 45 to 55 is probably the range that I normally buy in. I've bought SPF 100 and found it works as well as 45. I also read somewhere that everything over 55 was a marketing ploy and doesn't do much more.
But what ever SPF you use you HAVE TO REAPPLY. I normally do it every 3 hours. Sprays are easier to reapply and seem less messy in my bag (It still stays in a zip lock in case it starts spraying on its own). The other thing is that first application, I let it soak in before going out in the sun. So I'll put on the sunscreen and then have a cup of coffee and then go out and do things.