Initially I thought that you were looking for something to use at relatively short ranges and possibyly use "hold over" because you mentioned that term. Then I see you are talking out to 1000 yards so that changes things a lot. As a general rule you will find that in a given cartridge, lighter faster bullets will shoot flatter to somewhere around 500-600 yards give or take a little. Generally from there on out the heavier, higher BC bullet will quickly pass the lighter bullet and you will find that at longer ranges the heavier for caliber bullet, with a much higher BC, is going to get to 1000 yards faster, flatter and do it with much less regard to wind.
You mentioned 7mag so I guess you are talking about the 7mm Rem. Mag. This round with proper loads and bullets will easily get you to 1000 yards.
If you look "at this link you will find some free software that will help you".
Just remember that at longer ranges your hold over routine will not work but you will find a whole new world that will be amazing. The country beyond point blank range is fascinating.
Remember that drop is very easy to figure and account for. The wind, at longer ranges, is what will give you fits most of the time. That is where the heavy for caliber, high BC bullets shine. They buck the wind, drift less, retain more energy and shoot flatter at extended ranges and thus, normally give you better groups at extended ranges. There are exceptions to every rule but I will take a heavy for caliber, high BC bullet for extended range shooting any day over a fast light weight bullet.
In your comparison of the 120 grain bullets to possibly the 175 grainers you will find that if you are talking extended ranges to 1000 or beyond that the light weights will blow around like whiffle balls and drop like rocks, hitting like the lightweights they are. The heavier bullet, launched via a proper load, from the same gun, will kick butt when compared to the pipsqueak 120 grain pills.