There is little difference between the Mjollnir and the Sabre, excepting that the Mjollnir has more weapon hardpoints and increased cargo capacity.
Care must be taken with a Mjollnir filled with a full weapons array, partly because such a ship represents a huge financial outlay and partly because its weapon generator is smaller than the Sabre's with nearly twice as many guns to power. Nevertheless, a lethal adversary in a fight, shielded and armed on the level with weaker M3s.
During Reunion and the Terran Conflict the Mjollnir, like all ATF ships, is not available at any shipyard to buy and as a reward on completing the Balance of Power plotline.
In Albion Prelude it is available at Terran Shipyards.