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"...]