OK, I think I've got it (whew!). So my next question is: knowing this will help me...how? Is there some theory about progressions or voicings that suggest I should use a closed versus an open chord (or vice versa) under certain circumstances? Some nuance of mood or style?
(I know we're off on a bit of a tangent here, sorry. Maybe a mod can move this to a new thread if that makes more sense).
When I choose any particular chord voicing, I do it for reasons of sound quality, voice leading or playability. Whether it's classed as closed or open is completely irrelevant to me.
To expand a little on those reasons.
Sound quality - each arrangement has its own sound, despite having the same harmonic effect. For example, G7 320001 sounds quite different to the barre chord G7 353433, partly because the b7th is prominent as the highest note in the first case, but in the second case, that b7th is tucked inside on the 4th string surrounded by higher and lower notes, so is more subtly felt.
Voice leading - I might want the uppermost notes and/or the bass notes of a chord progression to follow a particular melodic line. So I'll choose my chord arrangement based on the shapes that give me that. This is where inversions are most useful - so we're not too far off topic.
Playability - Speaks for itself really. If a certain shape that I need is physically beyond my reach or ability in the time given, I'll compromise and simplify it. That may involve omitting non-essential notes, or re-arranging their order - whatever it takes.
