Harmonics to Noise Ratio Estimation
For large number of samples both will be indistinguishable.  

The Biased Version is the Maximum Likelihood Estmator (MLE) of the problem.
It means it has many nice properties for $ N \to \infty $.
The Unbiased version is Minimum Variance Unbiased Estimator (MVUE) of the problem.
Yet in practice when $ N $ is large, as in your case, it won't matter much and there won't be any difference between the two.

Yet, you should remember those are for the case the mean isn't known.
In case it is known, one should always use the Unbiased Form.