The fact that you are considering cancelling suggests that you don't really need one at the moment. If that's the case, I would wait. I upgraded because my old MacBook no longer met my needs, but there are some compelling reasons to hold off:
Prices of the retina MacBooks will almost certainly fall next refresh, and larger SSDs (at least on the base) are likely;
A 13" retina version is rumoured to be released, which may better meet your needs, and be cheaper;
Haswell (Intel's 2013 processor family) is expected to bring 30-40% faster processors, 200-300% better integrated graphics, and reduced power consumption to enable 10+ hour battery life.
With that said, it's not worth waiting for any of these features if it means struggling on for another year with a computer that is already slow. There is always something newer and better on the horizon, so you need to strike a compromise between meeting your needs now and not missing out on major advances.
Current information indicates that the earliest a Haswell refresh will take place is April, but more likely June. Buying now means your computer will be the latest and greatest for at least eight months, but if you can wait for the Haswell update, you probably should.