Today I was going to start reading Practical Common Lisp, but now that Arc has been released I don't know if it will be better to continue with PCL, and after take on Arc or perhaps start directly with Arc.
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.
Be cautious choosing Arc as a first lisp -- It's still in heavy development, so there may be unexpected sharp edges that we lisp veterans instinctivly avoid. You may prefer PLT Scheme (atop which Arc is written), as it's quite similar to Arc at the moment.
Do both, esp. if you have Real Work to do and Arc being a moving target will mess you up. What you learn in PCL will be a great help to understanding Arc, so you really are not doubling your studies.
