Learning Lisp: Start with Scheme, CL, or Arc?

I've never used any Lisp before but would like to start. PG's ANSI CL book looks good, as does Seibel's PCL book, but I've also heard great things about SICP. Then again, I see that Arc has a pretty sizable tutorial, and since it's written by PG, I'm guessing it's probably pretty good.

My free time is very limited, and my end goal is to write some interesting software over the course of the next 6 months or so -- and have fun doing it. :) I've only used languages like C, Java, Perl, Ruby, and Python in the past.

Any advice on which one to go with for someone brand new to Lisp? Does Arc come with enough tutorial material for someone brand new to Lisp in general?

Which is a bigger leap: to go from CL to Arc or from Scheme to Arc? Is Arc any harder to learn than the CL or Scheme?