Having been able to do a direct comparison between my Signature and george's series, I think the difference can best be shown using a photographic analogy. Think of a really nice well taken photo, perfect colours, perfect composition etc, the kind of picture you'd hang on your wall. That's the signature. Add a polarising filter and while it's the same picture, everything is just that bit richer. That to me is the difference between sig and series.
I'd also agree with jazzy that, for live playing, the signature (for me) is that much easier to work with although George would probably disagree having played his series for many years. I guess it's whatever you're used to.
Graeme