I learnt Ruby kind-of-alongside Rails and wish I'd spent more time focusing on the language itself; I'm going back and deepening my knowledge of it now. But it depends on how your mind works, whether you want to get a Rails project going yesterday, what knowledge you have locked away somewhere, and so on.
I've put in a bit of time in Ruby and it's opened up Rails (and programming in general) for me more than I thought it would. The biggest advantage, besides just general programming, is reading other people's code and documentation. Recently, I've been exploring i18n (internationalization) in Rails. By going into time_ago_in_words helper, I was led to the distance_of_time_in_words helper and it's source code (https://github.com/rails/rails/blob/bd8a9701c27b4261e9d8dd84...). By understanding enough ruby, the Rails code became a wonderful source of documentation.