Mechanically, MIC 6 has a Tensile strength of 166 Mpa vs 310 Mpa for 6061-T651. Yield strength is 105 Mpa vs 276 Mpa for 6061. Elongation at break is 3% vs 12% for 6061. Brinell is 65 vs 95 for 6061.
All aluminum is cast, but 6061 is a wrought alloy that is post-cast rolled or formed. There is also significant differences in the alloy composition as indicated by the Mpa numbers above.
Mn = 0.09-0.5 for MIC, Max 0.15 for 6061
Cu = 1.7-3 for MIC, 0.15-0.4 for 6061
Si = 0.05-0.15 for MIC, 0.4-0.8 for 6061
Fe = 0.26-1 for MIC, Max 0.7 for 6061
Zn = 1.8-5.3 for MIC, Max 0.25 for 6061
Mg = 0.9-1.9 for MIC, 0.8-1.2 for 6061
Basically, it is softer but more brittle (although not has hard, it still pretty much breaks before it bends - which is why it isn't used in structural applications). But it doesn't move around during machining or during heat / cool cycles as much though.