Typically 115's are cheaper and for plinking (as it sounds like your doing) cheaper is typically better since your not trying to shoot wings off a nat at 100 yards.
One thing to remember, though--different bullet weights have different ballistic characteristics. It's not wise to practice with 115 gr bullets then use a heavier weight bullet for self defense. That's part of the reason that the FBI and other federal agencies practice with the same round that they carry.