I have the HT 8x42, 8x54 and 10x54, but not the 10x42, so cannot really compare the two 10x binos. If it's any help, here is what I find noteworthy when comparing the two 8x models, 8x42 and 8x54. Many of my findings may apply more or less also to the 10x models.
First, the 8x54 model is considerably larger and quite a bit heavier: measured with eyecaps and strap, 1'117g, vs. 916g for the 8x42.
Then, interestingly, the 8x42 has more
usable eye relief (measured from the rim of the eyecup) than the 8x54, 15.5mm vs. 13.5 mm for the 8x54, although according to Zeiss' specs, both have the same technical eye relief of 16mm (measured from the surface of the eyelens). I measured with a precision Dynameter, so I am sure of my result (just for completeness, I measure 14mm of usable eye relief on the 10x54 model, so just a tiny bit more than on the 8x54).
The 8x42 has a minimum focus distance (measured) of 1.9m, the 8x54 has 3.1m.
For some further numbers, see my website at
Zeiss Victory HT 8×42 – Binoculars Today and
Zeiss Victory HT 8×54 – Binoculars Today
The 8x42 has a RFOV of 7.8 degrees, the 8x54 has 7.4 degrees. The difference is actually quite recognizable in practice, the 8x42 has not only a wider field, but similarly a wider sweet spot. Both are not great in terms of edge sharpness, but I find the 8x42 slightly better than its 8x54 sibling.
Central sharpness is excellent in both binos, the image characteristics such as color fidelity very similar. Both glasses ar very good at straylight suppression, a bit less good at CA control (at least for my eyes).
The 8x54 is of course brighter at twilight, the large 6.75mm exit pupil together with a 95%+ transmission provides a superbly bright image in difficult lighting situations (I suspect the 10x54 will equally "beat" the 10x42 model).
But the 8x42 with its 5.25mm exit pupil is still a wonderfully bright instrument, even at dusk or dawn, and you have to weigh the differences of the two instruments against the primarily intended uses.
Not sure this is what you were looking for. If the choice is between 10x42 and 10x54, I would personally choose the 10x42, despite the greater brightness of the 10x54, unless you are really looking for a "night bino".
fwiw
Canip