They are about as different as C for Unix, C for Windows, C for microcontrollers, etc. They're all really the same language; the biggest differences are the available libraries. Just pick a Lisp and go for it! :)

If you're on a GNU system, I found GNU Guile to be exceptional to learn Lisp with. Particularly with following the SICP lectures from MIT (with Hal Ableson and Gerry Sussman). There are lots of great options though!