Hi Amnon,
After thinking some more about your question, several facts stand out.
Most birders prefer 32mm over 42mm for its size and weight, unless very lowlight viewing is important. Many estimate that 42mm gives only an additional 20 minutes of use into dusk. Very few birders use 50mm, probably almost none for long birding walks.
Is this a fact? And if so, does it stand out? Most people over here (western europe) us 42mm. 32mm is used by people who bird less, or travel. It's not about the 20 minutes into dusk or dawn, but about the extra brightness you will expect EVERY moment looking through the bin. If you have dull light, drizzle, rain, or the bird against the sun, or a flying bird with bright sky, you are allways better of with more brightness. You will allways have a better view with a bin that lets more light go through!
I don't quite understand you are saying a lighter bin is better for long walks, while you have a brick like a trinovid around your neck?
When I carry my wife's 8.5x42 Swarovski EL, I am annoyed with its additional inch of length over my Trinovid, and fear that an uncapped lens could hit my belt buckle, something in the car, etc. The 10x50 Ultravid is another half inch longer still.
If you have your bins properly around your neck, the strap should be short and your bins shouldn't hang that low. I wonder what you want to say with something in the car?
And if you don't enjoy that, it won't be worth the money. And, I would bet, you don't.
A friend of me is birding for the last 7 years with this bin. He has seen 4500 species with this bin, and he is very pleased. If you are a hardcore birdwatcher, the 10x50 has only a weight disadvantage, but this is only small compared to the advantages of wider field and brighter image.
The Pentax ... ...main shortcoming is its small field of view. This is no handicap for distant birds, but takes some of the fun out of viewing star fields, and makes it harder to find one's way around in the sky. Still it seems like a very useful instrument.
A small FOV is allways a handicap. The human mind is focused on about 10 degrees sharp view (the other 150 degrees you see, but not sharp at the same time). If your FOV is 5, you cannot view through that bin the whole day. If it is 6 or more, you can call it good enough. If you have a FOV of more than 8, it's a pleasure to see through.
No single binocular, regardless of quality, can do very many things well. It is better to have a collection of binos that serve special purposes, even if each one is not of the very highest quality.
Whatever you buy, you always want to use the one that is brightest and sharpest. You won't use the ones with inferior quality, even if they have e.g. the quality of being lighter. The only thing that will come up in your mind seeing a distant bird is: I should have brought my other bins.