What's the difference between these two? Under what circumstances would I want to use one or the other?

The hi-build will do anything the lite version will, even provide a low build coating. The lite will require more applications to build up to the level of a normal hi-build application. Your technique is mostly to do with this.

Many guys who finish fly and light spin rods prefer working with the lite version and use more than one application if they desire a bit more depth.

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In my experience, the Lite version does not offer as long a pot life as the high-build version. Might want to give it a practice run first before trying it on that just-finished perfect wrap.
Clyde Dent

I like the lite version at least for a first coat. I try to get the tunnel full with it and get the threads just sealed, then I trim and put on later coats.

Probably a little easier to get a level finish with the high build after that. At least for me the lite has a tendency to sag in the middle and pull away from the ends when I get it too heavy (or sometimes when I think I have it just heavy enough to level well)

Doe's it hurt anything to use a first coat of FC Lite and then another coat of U40 LS supreme? This is the technique I was intending to try on my current project.

Why use two products? Put two coats of the one that you like to use