Definitively go for Squeak. It's a closed system in terms of the environment, or what you call the IDE, but it's fun to do webapps with - look for Seaside. However I always recommend everyone involved in development to take a look at it, just to understand how development in an image is working - and to experience a live system.
The main problem with Squeak, or maybe Smalltalk in general, is that once you get used to it, it's very hard to go back to the conventional way of programming.
Besides, I heard that you might become a better programmer if you work for some time in Smalltalk. I don't know if that's true, but I certainly like to think so.