I wonder what makes you say CL feels more well-engineered? I can't think of any reasons, unless you simply mean Scheme's under-specification.
Scheme is not just "conceptually" cleaner if you ask me. It's very concretely cleaner. The naming conventions for one ("rplaca" anyone?), the clear-cut orthogonal APIs, the numeric tower with exactness/inexactness, and [insert pun about "hygienic" macros being "cleaner"...]
That's one major issue with Scheme: sure, anything can be achieved in a Turing-complete language, but each Scheme does things a little differently. Common Lisp, meanwhile, actually standardised all that important stuff.
And while Scheme is conceptually cleaner, CL feels a lot more well-engineered as a language.