Paula Begoun is famous for saying that it should be after the moisturizer. I disagree, I'm with the before camp.
What you want, is the chemical sunscreen ingredients to sink into your skin and provide protection from there. They are useless laying on top of your skin, as they will wear out within an hour due to sweat, friction, etc. When you use physical blockers as TiO2 or ZnO, these need to be on top of the skin.
When you start off with moisturizer, you saturate your skin with lotion, and when you put on the sunscreen after that, the sunscreen ingredients (if you're talking about a modern sunscreen formultation with Tinosorb, Mexoryl, Uvinul A Plus etc.) which are very big might not penetrate the skin well enough, and stay on the surface of the skin.
The other way around: when you start with sunscreen and let it penetrate the skin, BEFORE you put moisturizer on top of it, the ingredients will all have settled in your skin. Paula Begoun talks about 'wiping sunscreen ingredients off' when you put moisturizer after sunscreen: BOLLOCKS. I don't see all my sunscreen getting out of my skin, and getting back on my fingers again. Nor do I see sunscreen dripping on the floor. The sunscreen stays in your skin, where it has been absorbed.
As for myself: I needn't use moisturizer during the day: the carrier lotion of the sunscreen provides more than enough moisturization. Perhaps you can try this too.
And on a side note: Paula Begoun has the worst sunscreen formulations, as her avobenzone (UV-A protection) is not properly stabilized as for example the Neutrogena with its 'Helioplex' is. Using Paula's Choice as sunscreen is the equivalent of using a paper umbrella. It's just very hypocritical of this company.