Mitsubishi HM vs GS systems – GreenBuildingAdvisor

For our new 1000 sq ft garage build, the HVAC contractor proposed using Mitsubishi MUZ-GS18NA/MSZ-GS18 units.  However, they ended up installing MUZ-HM18NA/MSZ-HM18NA.

From what I can tell, the specified GS system is 21.5 SEER2, while the installed HM system is about 18 SEER2.

Are there any other differences between these two systems? 

It also looks like the GS system is about $1000 more expensive than the HM system, so I would think that I should be credited that difference?