Hi Will,

Like you, I too struggled with this same problem of not knowing which to use on what item.
Finally I had one of those moments where the "penny dropped"...& now I'm getting consistent results more often.

FWIW here's how I look at various items & what I'll do with them.

Lines, Curves, Surfaces = Trim
Solids to Solids = Boolean

I know that the first thing everyone's going to say is "a Solid can be Boolean Diff with a Curve" & that's true!...
...the thing is there's always "Exceptions" to the Rule.

At the end of the day, what I really needed was to find some way of "thinking" that helped me remember what tool to use.
Solids = Booleans & everything else = Trim

Hope this helps you too, Colin