Sniper wannabe,
The choice of bases depends on the amount of internal travel your scope has and the required elevation needed to shoot the distances you are targeting with your rifle, bullet and muzzle velocity. Also depends on if you want a 100 yard zero and/or want to dial the entire elevation or hold the difference between what you a dial and what is necessary.
A 40 MOA base will give you 40 MOA, and approximately 11 MILs, of up travel by taking it from the down travel. Remember, if you want a 100 yard zero, the rise of your base and rings cannot exceed 1/2 of your scope’s internal travel.
As an example, I zero my 300 Norma at 100 yards with a Gen 2 4.5-27x56 Razor with 28.5 MILs of internal travel using a 50 MOA base, approximately 13.5 MIL, and easily have enough travel to dial out to 2000 yards using Hornady 225 ELD-Ms going 2950 fps.
I would recommend getting the 40 MOA base if your scope has the allowable internal travel.
Hope this helps...