The neck sized once fired brass in theory will produce better accuracy. The brass has less expansion to make to match the chamber dimensions and as a result the shot to shot variances are smaller.
In the real world will you be able to see the difference? If you have a super accurate target rifle perhaps, but even then I wouldn't bet on it unless you could compare 20 shot groups form both, and after 20 shots other variables come into play such as barrel heat and fouling, shooter fatigue, and weather changes on the range.