Hi Michael,
OK, I had an old original K5 and moved to a K3, so I will try to help a bit.
K5 is a good camera, and unless you are printing huge or cropping heaps then you will not need the step up in resolution.
Having said that the K3 with the AA filter off, and extra sensor does give you a lot more detail in the image - it is noticeable.
Other good stuff with the K3:
Screw mount lenses focus faster
The focusing overall is a lot quicker and more decisive than the K5, and overall you just get a much better focussing system
The rear LCD is a lot sharper
The bad/different stuff with the K3:
It is heaver - noticeably so in the hand - I really wish it was the same weight as the K5, as that felt 'just right' in my hands with a range of lenses. It took me a while - say 3 months to get used the the K3 in the hand. I don't notice it so much now, but I still wish it was lighter.
The batteries don't last as long as the K5 - again a noticeable difference
The colours (even in RAW) are different - different sensor, different way the RAW converter handles (I use Apple Aperture for this). I really used to like the K5 RAW colours I got, and again it took a while to get used to the difference with the K3. The advantages with the K3 are there is a lot more fine detail colour info recorded in RAW. Now that I am used to the sensor, and what it gives me - I will take images of sunsets that I wouldn't have bothered with the K5 - because I know the info is there, and I can pull it up in post. Even if I can't see it on the LCD. So - different colours that take getting used to (best way to describe is they feel flatter initially), but definitely more scope and 'headroom' with the K3
Bigger files - especially RAW. K5 were about 15MB, K3 are typically 25MB. This means bigger SD cards, and you will fill hard drives or SSDs quicker. Just need more spec on your computer really.
Finally - the 24MP sensor 'punishes' you much more than the 16MP sensor if you don't get it right - i.e. your technique has to be better, your focus better etc. etc (as a side note, I can only imagine what a 36MP or 50MP sensor demands out of you). Simply, when you get everything set right you will be stunned, if you get settings or focus a bit wrong you will be punished - I think this is a function of how good it can be when you get everything right, and how much you notice when it isn't like that - but be warned, it is a step up from the K5's 16MP sensor. The K3 has forced me to up my game a bit over the K5.
I think that is about it - it took me a good three months to get used to my K3 after the K5 and be really comfortable and happy with it. Having said that, I was in New York the day I brought the camera, and that day was about Manhattan, and got several stunning images which I am still particularly happy with.
I have had the K3 nearly a year now, and wouldn't go back to the K5 - the much better autofocus system, and what you can get from the 24MP sensor with no AA filter have spoilt me.
You will make your own decisions, based on a range of information. I wish you the very best with your decision making. In the end if you keep the K5 you still have a superb camera, and if you buy the K3 you will also have a superb camera. So you can't miss either way
Ross