Ask HN: Tester or Devloper? If Developer Node.js or Ruby?

I am currently a SysAdmin trying to break into developer career.. I have some knowledge in Python and developed few practice apps using it..

I have recently attended an interview with a company for a Software dev position.. After all the discussions they gave me two options

a) I can join in as a Automation tester for which they use Python, and work in there for few months until I learn the codebase and a development language they are using and move into dev position..

b) I have 3 months lead time to join.. So, pick up either Node.js or Ruby (which they use for development) and get familiar with it before joining..

So , what do you guys think? I have basic knowledge on Python, Should I master those skills in the three months and become a tester and move to dev later on,

or start learning the node.js or ruby and practice it 3 months, If so, which one to pick up?

pls advise..