The Maus was more heavily armed, and was faster. The T28 (above) had a 105mm cannon main armament while the Maus had 128mm main, and 75 mm co-axial gun plus the MGs. Plus as you said, the Maus had a turret, that's why it's a tank...the T28 above is not really a tank, it's a gun carriage that's why it was later named the T95 Gun Carriage. In other forces it would have been called a Self Propelled Gun.
The T28 was a poor 'beast' it was a needless extravagence, and they were foolish to think of the idea, it was just too heavy. The King Tiger is always said to be heavy, too heavy and it was only 68 tonnes.
Bridges wouldn't have been able to support either the Maus or T28, even the King Tiger had problems with some lighter bridges.