That's really for you to decide. Only you know how much you'll be using it and how much you can afford spending. If you'll be using the program a lot and can afford it, go for the CC version. If you're a student you can get a nice discount. If you haven't tried the CC version then why not do so and see what you think?
Honestly lets relate this to real life. I find it more expensive renting a car for 24 months than buying one. Interest, fees, etc.. But here they can let you drop out anytime i believe. If you plan on keeping this for over a year, then you can then just buy it, but it is honest;y your choice
Do you need the features that are there in CC or is CS 6 good enough for you? CS6 is a one time payment if you plan on using illustrator pretty much all the time and not just once every few weeks, otherwise go for CC.