Good question, @JamesStaddon! I haven’t really thought hard about it, but I think probably the Canon 70D would be the closest. A snapsort comparison shows that they’re pretty close. You have to bear in mind though, that the K-5 is 2 years older, so it’s all the more amazing that it can keep up with the 70D! Of course there are pros and cons for both sides. If you’re wanting to shoot video, the 70D is a no-brainer. Pentax isn’t known for video, they “focus” on photos, (which is how it should be, if you want a video camera, buy a video camera, not an SLR!)
I have previously compared the K-5 to your current camera, the 5D Mk II. Again, it’s not really a fair comparison, the 5D Mk II is full-frame, but it’s also 3 years older than the K-5. The K-5 wins that round also.
Next, I compared it to Canon’s “latest and greatest” announced only a little over 6 months ago, the 7D Mk II. The 7D II is an awesome camera, I was impressed! (Until I thought to compare it to the K-5) 🙂
However, the K-5 finally met it’s match in the Canon 6D, another full frame camera that is one year newer than the K-5. If you go down through the list of advantages and disadvantages, it’s basically the same, as compared to the other Canon cameras. The K-5/K-5IIs comes close enough that I think I’ll keep my Pentax. 🙂 For one thing, it’s close to 1/2 the price…
As I said, I started out high, so if you’re interested, make me an offer!