Common Lisp, Scheme and Arc are similar to each other in many ways, so you won't have many problems if you change your mind. One of the exceptions is the macro system: CL and Arc are similar ("raw" macros) while Scheme takes the hygienic approach. Understanding a macro system and learning its idiosyncrasies takes a while.
I think "On Lisp" makes some very valid points in favor of the Arc's approach, but probably you'll find one system more intuitive than the other and it will boil down to a matter of personal preference.
For these reasons I think the book is more important than the language. Find a book that resonates with you and learn. You'll be in a better position to decide then.
