The K5 is hardly worth anything these days (I see them selling now for less than 300 here in the forums...) so you might as well keep it for now. If something goes wrong with the K-3 and you have to send it in for service, you'll have another camera to play with in the meantime. So, keep it as a 2nd body. Also, take it to places where you wouldn't want to take the more expensive camera (e.g. the beach, hikes, canoeing, etc).
I have two bodies for similar reasons, and more: I use the K20D for more serious shooting (I still think the IQ is great at low ISO), and the K-r goes to places where I or my wife will have it around the neck for a long time. K-r + Plastic Fantastic 35 2.4 = featherweight
so trading the K-5 for something like a K-S1 would also make sense - if this idea of a small light camera plus a light prime appeals to you.