An ultrafast structural, Jahn-Teller (JT) driven, electronic coherence mediated quantum dynamics in the CH4+ and CD4+ cations that follows a sudden ionization by an XUV attopulse, exhibits a strong isotope effect. The JT effect makes the methane cation unstable in the Td geometry of the neutral. Upon the sudden ionization the cation is produced in a coherent superposition of three electronic states that are strongly coupled. On the ground state of the cation the few femtosecond structural rearrangement leads first to a geometrically less distorted D2d minimum followed by a reorganization to a shallow C2v minimum. The dynamics is computed for an ensemble of 8000 ions randomly oriented with respect to the polarization of the XUV pulse. The ratio, about 3, of the CD4+ and CH4+ autocorrelation functions, is in agreement with experimental measurements of the high harmonic spectra. The high value of the ratio is attributed to the faster electronic coherence dynamics in CH4+.