I have owned a couple pairs of the 10x42s and have relatively good experience with the 8x42s. My advice would be to go with the 8x42 configuration. Here is my rationale:
1. 8x magnification provides a larger exit pupil (little round circle of light in the binocular's eyepiece when the binocular is held out in front of you) which allows more light to reach your own pupil thus giving you a brighter, more contrasty image. The larger exit pupil also makes it easier to hold the image in front of your eye since there is a larger surface area to compensate (5.25 mm versus 4.2 mm).
2. 8x magnification provides a better depth of field. Depth of field is the distance in front of and behind the main focus point that the image continues to remain in focus. For example, lets say you are watching some does feed 20 yards from your stand. Then, a buck walks out at 35 yards. With an 8x binocular the chances are slim that you will need to greatly refocus, if at all, for the change in distance. With a 10x binocular you most undoubtedly will have to.
3. In several tests done by reputable binocular manufacturers it was found that higher magnification does not necessarily allow you to see more detail (within a small, given range...7x-10x for example and assuming handheld). What does allow you to see more detail is the quality of the glass being used, the quality and number of chemical coatings on the glass and the reduction or elimination of visual distortions such as chromatic (color fringing) and spherical abberation (bending of the image along the edges).
4. As you have noticed, a 10x binocular is much more difficult to hold a steady image in. Imagine trying to hold place an 8 foot long stick into a small hole versus placing a 10 foot long stick into the same hole. Though it isn't an exact analogy the process is still the same.
I have had the opportunity to use a pair of the 8x42s and must say that they are a very good buy considering the price and the optical performance that they provide. I had a pair of 10s that served me very well for about a year but when I bought a second pair (the new camo version) I found that the spherical abberation in them occupied better than 1/3rd of the field of view....and subsequently returned them.
If you want some further, excellent information on binoculars in general, and in some cases the Monarchs in particular then please take a gander at the following links. There are some excellent reviews in some of them.
www.optics4birding.com
www.betterviewdesired.com
www.birdforum.net
www.opticsplanet.com
Hope this helps with your decision.