For a Lisp user the s-expression variant would be more useful, because it is easier to write and read. It also helps that it is easier to indent / pretty print, because the usual pretty printer algorithms can be used. For LOOP one would need an indent / pretty printer with either some heuristics and/or knowledge of LOOP grammar.

Personally I use LOOP because it is built in. But generally I prefer something like ITERATE, which is similar powerful but fits better into the host language (-> Common Lisp).