I have owned the Remington 700 Mountains rifle a couple of times, both in 280 Rem.
Finally got a 280 Rem in a LH Rem 700 (XCR action).
On all 3 rifles, over the years, I have gone with the Leupold Vari X III, VX-III and now VX-3, all in the 3-10x40. The current 3-10x40 has the B&C reticle.
Light, bright, robust, and w/ the B&C reticle I can adjust point of aim quickly should the animal move or be found at a different distance than anticipated.
I also own a few of the 2.5-8x36 scopes w/ B&C reticles. Great scopes and just a little shorter and lighter. Use them on SA rifles, whereas I am using the 3-10's on LA rifles. As these rifles are in less powerful cartridges, the maximum of 8 power is not a handicap for hunting.
I have owned the 6x42 in the past, and while a great scope for more open country, I switched back to the variable, as I hunt varied terrain and the variable provides better service by being able to adjust the power range down for short distances, and dialling up for longer distances, and range work where I appreciate the 10 power to see my target better when practicing, or doing load development.
The main advantage of the 6x42 over the 6x36 is the extra amount of light that gets to your eye under low light conditions; secondary becomes field of view. But this is only if you are younger and your pupil will still expand to 7mm. As we age, our pupil can no longer open to 7mm, and 6mm may be fine. At about 50 years old, it will only open to about 5mm, so the larger scope will longer make a difference, other than field of view.
The main reason I prefer the 3-10x40 is that I have that extra edge in magnification as stated above, and a little better field of view, and fits LA rifles such as the 280 Mtn rifle just a little better. Being in my 50's now, my pupils will not open as wide as they once did, so when hunting in low light conditions, I do not crank it up to 10 power anymore for longer shots, as 8 power will transmit all the light my eye will accept. And at 3 power, I can still find an animal in the thick brush at close range, where a fixed 6 doesn't provide the same versatility. (And yes, I have shot mountain goats at 15 and 30 yards in the timber and stunted evergreens on mountain tops, so being able to dial down is an advantage that may be required!)
I have purchased a Ruger No.1 in 257 Roberts and have been thinking that a fixed 6 (that aforementioned S&B Klassik has my attention) mentioned would be nice on it, as it would mainly be used for open terrain hunting for deer or antelope.
As most of the large open fields that I have permission to hunt locally have access through the timber, a variable makes sense where I sometimes catch the deer in timber as I make my way in and out of the fields, as this is the majority of the hunting I would have for this rifle, whereas the antelope hunting occurs less frequently. I am also planning an axis deer and blackbuck hunt for this rifle.
Decisions, decisions! (If I could locate a VX-III or 3, 3-10x40 B&C in gloss for this beautiful No.1...)