You know... When I got interested in Lisp I also thought I'd have to choose. Then, after reading and asking a lot, I ended up choosing Common Lisp. Not because it's "better" than Scheme in an absolute way, but because it had features I knew I'd probably use in the next 2-3 years in my projects:
- A very efficient compiler for numerical computation (SBCL),
- The condition system, and...
- I have to admit that SLIME also played an important role in this decision. :-)
So if I were you, I'd start learning a tiny bit of each language, and find out what are the relative strengths and weaknesses of each (I mean, those related to what you'll need/want to do most often) so you I could pick one. You may just find out that you'll learn the other later also (happened to me -- I had to learn more Scheme after one year studying Common Lisp)
As to Arc, Clojure and Lush, and don't know those. I suppose the same argument could apply to them too... The reasons I didn't want to invest too much on them were: Arc does not seem to be interesting for numerical computing, and P. Graham seems obssessed by code size (to the point of making his code unreadable); Clojure seems nice, but I don't like the JVM. And Lush... Well, besides using dynamic scope, the compiled and interpreted versions of the language are very different -- and you can't compile recursive functions. So, it was either Scheme or Common Lisp to me.
