I used to have a .270 that I used for predators and what-not. I had a straight 12 power scope on it and during deer season, a large buck ran out about 20 feet away and when I shouldered my rifle all I saw was fur bouncing around.
You need to be certain what caliber you are going to use and just what you are going to use it for. If you have enough money to have one specialized rifle for each of six different things, then, by all means, put the 20X on it - as then you can use one of your other rifles with a lower-power scope for close-range hunting.
If you DON'T have money to have a different rifle for every purpose, then go with a VARIABLE, preferably in a low-magnification range of about 3-9 (or 3.5 to 10 at most) and be happy with it.
A major question you must put into the equation is:
Is the subject rifle even capable of such tight groups out to 500 yards? If not, then don't you think you should keep the power down and keep the shots within a reasonable distance, say within 150 yards?
A friend of mine has an old beater WWII sporterized bolt action Remington 30-06 and I asked him how far he wants to shoot deer. He looked out at the field and told me "500 yards!"
As it was, at the range, he couldn't even shoot an 8" diameter rock at 50 feet with the thing, OFF THE BENCH! He wasted 20 rounds of ammo and left, not even having the Lyman open peep sight sighted in yet!
I told him he had better buy several hundred dollars worth of ammo and start shooting EVERY SINGLE DAY, then by deer season, he will be proficient enough to MAYBE drop one at 500 yards off-hand.
Other than cross-wind, shooting longer distances is not difficult to do, anyone can do it -- even with open sights.
When the 45-70 was used in war, after enough shots, the shooter gets to know just how high to HOLD the shot in order to make a hit at 300, 400, 500 yards and beyond! So, with enough shooting and practice, and, perhaps some ballistic software to help you, and, perhaps a chronograph so you know the speed at which your bullets are flying so you can make accurate hits based on accurate trajectory information. Of course, in the olden days, before software and chronographs, the shooter's only way to achieve long-range accuracy would have been to just shoot, shoot, and shoot some more!
When I was a child, I had a BB gun. I must have put OVER 10,000 BB's through that thing! After some time and a LOT of shooting, I got to instinctively KNOW just how much to hold over for a shot out to 100 yards! For a $29 Daisy BB Gun, 100 yards is a FAR SHOT!
I was sighting in a 50 BMG bolt action. The scope was a shimmed Leupold Mil-Dot Mil-Spec scope (worth about $1,600.). I was trying to get the scope on paper at the 200 yard range. I couldn't tell where I was, even when I moved the target in to 25 yards! Finally, out of frustration, the huge 150' high berm beyond the targets (which serves as the backstop for all the bullets) I picked out a rock about the size of a basketball on the lower portion of berm and shot at it. Immediately I saw my problem. With the scope dialed all the way down, as far as it would go, I was still hitting about 36" high as I saw the hole in the dirt berm about 36" above that rock! So, I knew immediately, at the extremely close range of 200 yards, I must hold about 36" lower than what it is I am trying to hit. As soon as I knew that, I was able to hit every rock out there! The owner of the gun (it is not my gun) ended up removing the shim. Around my parts, there just aren't enough places to shoot 1,000 yards!