Each implementation has its advantages. The Open Dylan compiler is faster than Gwydion Dylan's compiler, but Gwydion generates faster code. Open Dylan has a very nice IDE, but the IDE doesn't run on Linux yet. Gwydion runs on more platforms, but Open Dylan has better library support. We're working to fix these discrepancies as fast as possible. In the meantime, both compilers support the same core language and a nice set of common libraries (common-dylan), making it fairly easy to use whichever compiler suits your needs for a given project.
There is also a byte code compiler called Mindy which is still supported but no further work will be done on it. All future work will concentrate on the two compilers. Although we are working to make code more portable between compilers there are still some libraries that only work with one or the other.