I would think the biggest issue is the turrets for windage and drop compensation, those are the ones that stick out from the scope. The magnification, if adjustable, is normally a ring just in front of the back end, so no issues there I would think.
I voted 10x42, as the extra mag is very useful for long shots on the hills. The 8x56 is too big for hill work, but outstanding in low light. If you're just hill stalking, you may not need the extra bulk of an 8x56.
I wouldn't go for a fixed power for anything other than dedicated short range (ie woodland) work.
Longer distances = variable power!
If "fiddling" about with variable power was such a hassle and got in the way then why do our boys (and the yanks etc) in the Kush and the desert invariably go for variables, often high zoom, and their lives not just the game's depend on it?
I never had a problem with variable power in 10 years shooting for Her Majesty or in 30+yrs shooting for myself.
Rejecting variable power on a scope is a form of inverse snobbery, something which is all too common in this sport in my opinion.
That's my tuppence worth anyway, striker hit on can of worms and standing well back.
I voted for the 6x42, because out of the choice given that one does me for the ranges I shoot at. I have both fixed and variable scopes on my rifles and do not really have a preference for either. However, as the question asked for a choice between specific scopes I made my choice accordingly.
I voted for the 6x42. I have 2 of them 1 on my .270 and the other on my .243. I have had them for years and never felt the need to bring deer or foxes in any closer by using a bigger magnification.
I don`t like variable scopes on stalking rifles, but that is a personal thing, too fiddly, especially when you are shooting quick.
You also have to think about the amount of light that you take into your pupil so anything that gives more than 7mm of light is wasted. so 8x56 or 6x42 is the ideal combination as these are multiples of 7.
Stick with the 6x42 - all the power you need for a long shot - and have taken several to 200 to 250yds which is as far as I want to shoot on the hill or at a fox.
Have used the 10x42 Schmidt and Bender - not nearly as bright as the 6x42 - definately an issue when Hind Stalking.
I have also used variables - I just leave them at 6 power. If I ever buy a variable, I would be looking for one that is very low powered at the bottom end, rather than very powerful at top end.
But I do like to be able to see the whole deer, plus what is around it through the scope. I have used powerful scopes on friends rifles and I find them, a) too slow to get on target, and b) frustrating to try and follow a moving beast - ie just been spooked and you are following anticipatig it pausing.
But do make sure your centre reticle wires are not too thick - I have a Meopta which has thick outer posts - great in low light, but fine centre so that it doesn't obscure target.
One issue with only 6 power is you can't quite see your bullet holes on a target at 100 yards, so you either need a spotting scope, or you have to walk down range between groups.