The magnitudes of Gt and Gtf are identical. However, the phase of Gtf provides a better match to the phase of the continuous-time system through the resonance. As the frequency approaches the Nyquist frequency, this phase match deteriorates. A higher-order approximation of the fractional delay would improve the phase matching closer to the Nyquist frequencies. However, each additional order of approximation adds an additional order (or state) to the discretized system.
If your application requires accurate frequency-matching near the Nyquist frequency, use c2dOptions to make c2d approximate the fractional portion of the time delay as a Thiran filter.