Mini M: 14Lx8Wx10H (5.5 gallons) ~$105 shipped
36-L: 15Lx9wx11H (6 gallons) ~$65 shipped
The 36-L seems like a much better deal. Not only does it hold more water, but it is also almost half the price of the Mini M. Both are clear glass, though. I guess the major decision point here would be the brand name.
Take a look at the measurements on both sites again. Seeing how both tanks were made in cm, when converted to inches, one of the sites rounded up. According to their measurements in cm, they are the same tanks.