If you look at performance benchmarks over the last few years, you don't see the same kind of jump in performance with each processor generation as you did years ago. That could change later this year when Intel releases their Skylake chips, since they'll be the first real significant architecture change in quite some time.
That being said, it depends on what prices you can get for the k and non-k versions of the CPU you're looking at. Typically, the difference is quite small, and if it were me, I'd go with the k version, just because you may decide to overclock later if you want to get some longevity out of your platform. If you're not in any hurry, I'd even wait for Skylake to come to market, which is within a few months and see what kind of pricing you can expect and how the benchmarks come back for them.
CPUs can last a long time. My secondary system is a Core i7-940 that I've been running day and night for about 7 years now, overclocked, and it's still going strong. At an office where one of my friends works, they're still running desktop servers on Pentium II chips (yes, you heard right, Pentium II) that are still kicking along 20 years later. So they can last quite long.
I'd look at it ultimately in one of three ways - either go with a higher end platform now that you'll have a few years from now; if you're in a hurry, go more mid-line and possibly upgrade later, or just wait until Skylake is released and make a decision then. No matter which route you go, it's never long before the next generation is out, so it's forever a game of catch up.