When comparing Guile vs Gambit, the Slant community recommends Gambit for most people. In the question“What are the best scheme implementations?” Gambit is ranked 3rd while Guile is ranked 4th. The most important reason people chose Gambit is:
Gambit is fast/efficient, you can see benchmarks [here](http://gambitscheme.org/wiki/index.php/Gambit_benchmarks).
Ranked in these QuestionsQuestion Ranking
Pro GNU's official extension language
Chances are a system running enough GNU software will already have it installed.
Pro Easy to customize and extend applications
Guile makes it easy for users of your application to write extensions without needing to understand the plumbing of your program.
Pro Good performance
Gambit is fast/efficient, you can see benchmarks here.
Pro Can be easily embedded into an existing C/C++ codebase
Gambit has a built-in compiler that generates C code, which then is passed to your system's compiler which in turn compiles it to native code. This makes it easy to integrate Gambit into existing C/C++ projects.
Pro Very portable
It is very portable as it has no external library dependencies. It will build as long as the platform has a C compiler.
Con Weak copyleft
Con Lack of SRFIs
Gambit natively implements few SRFIs. Additional SRFIs are available through the Black Hole and Snow third-party systems.
Con Documentation is poorly formatted
The Gambit documentation directs you to further resources, depending on what you're looking for. Because things are separated into different places, it can be difficult to find what you need.
Some documentation is only available in HTML or PDF formats that are hard to read/follow.