Hey Wave Dancer, welcome to the forums!
I'm Alex.
I have the K-3 and while I don't have the K-50, I own the K-30 (nearly identical to the K-50). I would trust them equally, to be honest. There's merit to the thought that the K-3 will handle more abuse (it is certainly a little tank - I feel like I can drive nails into lumber with it). But it's a bit bigger and certainly with more mass than the K-50.
If I were you, and you don't need critical features of the K-3 (uber low-light AF sensitivity, 27 AF points, C-AF tracking, dual SD card, 8.3 fps, 24 mpx resolution, etc), then I would go for the K-50 instead. It should be plenty durable and just as weather sealed. I have abused my K-30 and it is still as good as new almost two years after getting it. First, I used it while in a 'combat camera' role in Afghanistan:
Top - K-30 + DA* 16-50
Bottom - K-5 + Grip + DA* 60-250
I don't know the extent of your trekking, but if it's serious and you will be gone a while, for the price of one K-3, you can get two K-50's. That should certainly suffice to have a backup in the event of a failure and you are too far removed to get a replacement. Also, the K-50 offers AA battery compatibility that you may find very handy if you are in some 3rd-world country that doesn't have readily accessible electricity, whereas AA batteries can be found literally anywhere.
Or, if you just go with one K-50, the difference between that and the K-3 will allow you to splurge on an excellent weather sealed lens, either the DA* 16-50 or the HD DA 20-40 Limited WR.
For expected quality of the K-50, see
this photo essay that I compiled using the K-30 when I went to New Zealand. There's 82 images involving lot's of hiking (roughly 100km worth), so grab some popcorn - it'll take a minute to digest all the content there
You may want to compromise and get a K-5 II/IIs for the improved low-light AF and magnesium body without the (very slight) added bulk of the K-3. Here's another photo essay of when I conducted a
military training exercise in Finland this past winter (40 images). All those images were with the K-5 IIs and the DA 18-135 WR.
Also, buying the K-50 will allow funds for other stuff, and one of which I recommend is a tripod. The
Sirui T-025x is my recommendation, and here's why:
Can you imagine fitting a tripod in this camera bag?
How about if you look under the flash and cleaning supplies?
Since getting it, I've taken it all over the place and it fits inside my hiking backpack anywhere I want to stuff it without competing for space with my clothing, food etc. My preferred way to carry it while hiking is this:
I've hiked countless miles with it on my belt loop and never had an issue - I often forget it's there. If that's piqued your interest, you can see Pentax Forums' full in-depth review of it
here.
This is the most recent serious long exposure I've taken with it - 30 second exposure, but there are over 15 quality examples in that review on Page 9 - Real World Usage.
So yea, long story short - get the K-3 if you need some of the critical features, otherwise the money saved by getting a 'lesser' camera will be better spent on accessories/lenses. Ultimately I wouldn't worry about any of Pentax's cameras being more weather resistant than the others - I've beaten all of mine to hell and I see no difference in any of their ability to keep back the elements.
Hope this helps
-Heie