It won't be until Q2 that they release a refresh. But if you went through life not buying anything for fear it will be outdated in a few months you would never buy anything.
I doubt they will be any noticeable changes to asthetics anyway. They could remove the DVD drive and use SSD but I HIGHLY doubt it. And there won't be blu ray as Jobs regarded them as outdated. So updates will most likely be hardware and in my opinion the current hardware will last a good 5 years before the system starts to struggle on more demanding software.
Oh and Im sure allot of people will disagree; go for the 1440x900 resolution. The 1050p makes text way to small and having to zoom all the time was a pain; I have good eyesight btw. I say this because I assume this is the reason you are upgrading from 13 to 15?