Strumming requires highly developed technique, like anything else. I know players who can perform any kind of strumming you want with a very thick pick. I have seen less experienced players accomplish the rhythmic aspect of strumming more easily with a very thin pick ---BUT the tone they produced was awful from a recording point of view (think playing card in bicycle spokes).
So, does that mean thick picks are good and thin, bad? No, but I think the judgement that thicker tends towards a more substantial tone and thinner tends toward less tone and more "click/clack-iness."
My goal when recording acoustic guitar is to capture the sound of the guitar that is appropriate to the context of the musical presentation which can vary greatly:
- Rock band w/extra guitar (acoustic, heavy strummer)
I will almost always ask a strummer with a thin pick to try a thicker pick or one of a different material. I keep a tin with dozens of types of picks at the studio.
jay