Good questions, I'm a beginner too but from what I understood:

• r6rs or scheme are minimal in language specs but not as portable as cl. The libraries for Chicken Scheme won't work in Racket for example. But pretty much all libraries in CL will work on almost all of its implementations (SBCL, CCL, ECL, etc)
• SBCL compiles to native code but Clojure compiles to JVM bytecodes. So SBCL is faster performance-wise. However, if there is a library in Java world that you particularly like, Clojure would have better interoperability with it.

Do a search on this subreddit for other views. This comes up often.