IME, a lot of the decision depends on the quality of the glass in the scope. My personal favorite is a Schmidt Bender PMII 4-16 X50. I have been shooting Marmots out past 1100 yards with it this year. The scope offers such great resolution and contrast that even little targets like this are easily seen. I have an IOR 6-24 on another varmint rifle. It's a really good scope, but not an S&B. I seldom use it at 24 because mirage becomes so bad I can't see through it. I use it mostly around 16, and sometimes up to 20 if it's cool enough outside.
The Monarch is a good enough scope, but there are other much more noted for superior turrets than the Nikon. SWFA makes a like of moderately priced scopes that are known for accurate and repeatable turrets. The Nightforce 5.5-22 would be, probably, the best compromise for your use. It's not cheap, but still close to half what a S&B, Premier, or US Optics will run, and the turrets are absolutely reliable. Very important on a long range rifle. Spend the bucks now and save yourself a lot of frustration. If you are serious about long range, you'll end up with a better scope than the Nikon anyway.
