Master’s in computer science or free online classes?
What OBu said: This is going to depend on what your goal is. 


If you want to work as a programmer, generically, then a MOOC might be a good way to skill yourself up. As far as studying, fundamentals are going to pay off no matter where you work: Algorithms, Software Engineering, Math, etc. The problem with MOOCs is that it's not clear if they are going to get you through the door when job hunting; so be sure to focus on creating work that can demonstrate your skill, and help get you over that barrier. 
If you want to create research, write papers etc, then you could always try to create your own research program. Having said that, if you don't have an established history of auto-didacticism then this is going to be difficult to maintain. A real masters program helps guide you down paths, and force you to prune those search trees.
If you want to work as a programmer, in the NLP/MT/AI fields, then you might have to do both: MOOCS and an MSc. 


A side note; be warned that there are lots of MSc. programs out there, varying in style and quality. Some are essentially just more courses, while others are intended to produce new research. Be sure that the MSc. program you register for is going to get you what you want. Do your research.