Abstract : Domain Specific Languages (DSL) are becoming increasingly more important with the emergence of Model-Driven paradigms. Most literature on DSLs is focused on describing particular languages, and there is still a lack of works that compare different approaches or carry out empirical studies regarding the construction or usage of DSLs. Several design choices must be made when building a DSL, but one important question is whether the DSL will be external or internal, since this affects the other aspects of the language. This chapter aims to provide developers confronting the internal-external dichotomy with guidance, through a comparison of the RubyTL and Gra2MoL model transformations languages, which have been built as an internal DSL and an external DSL, respectively. Both languages will first be introduced, and certain implementation issues will be discussed. The two languages will then be compared, and the advantages and disadvantages of each approach will be shown. Finally, some of the lessons learned will be presented.