Well, let's run some numbers, and see what we can learn:
If you're a level 1 Variant Human Fighter with 16 Strength, Great Weapon style, GWM, and a greatsword, your value for X is 11.33 (4.16+4.16+3), and your base to-hit bonus is +5. You gain DPR using GWM in cases where you need to roll a 10 or lower on your d20, which equates to attacking an AC of 15 or lower. Against a target with 16 AC or higher, it's more effective not to use -5/+10
If you're a level 1 Variant Human Fighter with 16 Dex, Archery Style, Sharpshooter, and a Longbow, your value for X is 7.5 (4.5+3), and your base to-hit bonus is +7. You gain DPR using SS in cases where you need to roll a 12 or lower on your d20, which equated to attacking an AC of 19 or lower. Against targets with an AC of 20 or higher, it's more effective not to use -5/+10
If you're a level 5 Variant Human Barbarian with 18 Strength, Rage, GWM, and a Greatsword, your value for X is 13 (3.5+3.5+4+2) and your base to-hit bonus is +7. You gain DPR using GWM in cases where you need to roll a 9 or lower on your d20, which equates to attacking AC of 16 or lower. Against targets with an AC of 17 or higher, it's more effective not to use -5/+10