SPF 50 blocks 25% more UVB than SPF 40 does. Measuring it as percentages makes it non-linear in a way that most people find confusing. Imagine we had one sun cream that blocked 99.9% and another that blocked 99.5%. Sounds like nothing; only an 0.4p difference, but is actually 5 times as effective.
You're right about how long it lasts also being an important factor. UV-A protection is also another very important factor. But as someone with pale skin even by Scottish standards, the difference between SPF 40 and SPF 50 around noon is significant, even through I consistently re-apply every hour. I won't get burnt, but I'll end up with more sun damage - and that lasts until late autumn.
You're right about how long it lasts also being an important factor. UV-A protection is also another very important factor. But as someone with pale skin even by Scottish standards, the difference between SPF 40 and SPF 50 around noon is significant, even through I consistently re-apply every hour. I won't get burnt, but I'll end up with more sun damage - and that lasts until late autumn.