I saw that offer too on the website and contemplated that myself. Thing is you'll need more than just the 55 to make the most out of it. It'll blow your budget out a couple times over just getting the right glass for the beast of a camera. The K-5 IIs on the other hand means you get more bang for buck and can devote the spare cash on high end glass you could use for future K-mount cameras. Even a FF one.
But seriously, it's all about what you intend to do with the images thereafter. The 645D is an awesome camera, and still very portable and hand hold-able. The DR and VF are something to behold, but it has slower shooting and processing speeds - something's gotta give. The K-5 IIs is responsive but the VF paltry in comparison. Once you get spoilt with such a VF as the 645D you'll probably not even settle for a FF camera anymore.