Guile is designed for embedding. But it is vanilla Scheme, which might not be as modern as the Fennel flavor.
Maybe, the Fennel all gets lowered to Lua at build time, so you don't need a (maybe more expensive) Guile runtime? Or, Lua has a more comprehensive library ecosystem?
If I recall correctly, you can do both. You can compile your fennel code to lua, or you can use fennel as a library in your lua application so it will understand your fennel code.
Guile is designed for embedding. But it is vanilla Scheme, which might not be as modern as the Fennel flavor.
Maybe, the Fennel all gets lowered to Lua at build time, so you don't need a (maybe more expensive) Guile runtime? Or, Lua has a more comprehensive library ecosystem?