In the search for neutrinoless double beta decay ($0νββ$) experiments, common methods for sensitivity calculations include the counting method and the spectrum fitting method. This research compares their difference in sensitivity under various energy resolutions. Additionally, the performance of high and low Q-value $0νββ$ isotopes is compared. The results of this research could provide guidance on the choice of methods for sensitivity calculations, energy resolution and $0νββ$ isotopes for future $0νββ$ experiments.