I do not know Lisp but heres why I think PLT Scheme is a good choice:
Really Good Documentations
http://download.plt-scheme.org/doc/html/
DrScheme
DrScheme is an amazing programming environment that comes with a compiler, stepper, syntax checkers, REPL, myriad of languages support and is extensible. This makes the entire experience fun. See some screenshots
Good Support from the community.
The community is very enthusiastic about their language and very supportive too. If you have any questions, mailing lists, groups and forums are available. Even the authors are very very accessible.
Continuing Research
PLT is very active and continue to make their tool better and better. You can expect a lot of innovations from the labs. eg. Im pretty excited about using this: http://blog.plt-scheme.org/2009/05/typed-scheme-20.html
Free Books and interesting tutorials to get you started.
http://www.htdp.org/
http://world.cs.brown.edu/
Good modules and extensive libraries.
http://www.cs.utah.edu/plt/develop/
http://planet.plt-scheme.org/
also see docs
If you are learning Scheme, give DrScheme a try.