I was doing data analysis, In that I got two errors by two methods, I want to find out the best method in terms of accuracy, I got two errors, +0.1 by method x and -0.1 by method y, I want to know which is the best method in terms of accuracy?
If your example is that one, the relative error in the first case is 0.001/300 and the relative error in the second case is 0.001/299.9 (slightly bigger).
In any case, a qualitative insight would reveal if this difference is really a meaning one or not. Not everything in this world is numbers.
If I were to choose, and overestimating and underestimating had the same penalty, I would choose the simplest method (easier to explain and faster to compute).