What duh heck (Wdh) is an Untravid? Is that what they renaming the new Trinnies? That sounds about right!
Couldn't resist. The ultra leicables will answer your questions shortly, but one thing's certain, if you're looking for a compact pair, the 50s are out. The 32 model is more compact than any other top tier bin in its class.
As far as your question if a 10x would be hard to steady, it will likely be harder to steady than an 8x for most users. How much more? That's a question you won't be able to answer until you A/B an 8x and 10x Untravid. If you shake too much, the extra reach you hope to gain with the 10x might be lost, and you'll see more detail over time with an 8x.
For normal birding in seasonal months, the 32s should be as bright as the 42s. On really dark overcast winter days or at dusk any time of year, the 42s will beat the 32s, and they all will beat the 10x32 model with its 3.2mm exit pupil. It's a math thing. Ask Henry.
There are always trade offs, if it's not one thing, it's another. As I always like to say, binoculars are a set of compromises (except, of course, the 8.5x SV EL for those who are not sensitive to "rolling ball"
.
You might find the 8x32s are too small for your hands. Or you might find the 10x42s have too much chromatic aberration (false color). In general, the longer the bin, the better controlled the CA in the same series. So 50s would be best in that regard if you're sensitive to CA, but then you have the extra weight to contend with.
Do you wear glasses with binoculars? In general, Leicas are not the best in that dept.
While you're waiting for members with actual field experience with the Ultras to chime in, check out the Leica reviews on allbinos to get an idea of what somebody's got to say about them (YMMV).
http://www.allbinos.com/183-binoculars_review-Leica_Ultravid_8x32_HD.html
http://www.allbinos.com/216-binoculars_review-Leica_Ultravid_10x42_HD.html
http://www.allbinos.com/194-binoculars_review-Leica_Ultravid_10x50_HD.html
Brock