Both amps have been well reviewed. The 7500 is more expensive, due at least in large part to being built in the US.

I don't know that the 7500 has any more ground loop issues than other amps. There have been several reports of folks having trouble with ground loops, but that's not indicative of an inherent problem - ground loops are a nasty, obnoxious, sometimes very difficult fact of life for stereo and home theater gear. My 7500 has never had ground loop problems, and has been a great amp for me.

This spring, the MPS-1 was having problems operating with 4-ohm loads (reported by owners of both AV123 Reference speakers and I think some Polk owners). I never heard if a solution was implemented. You may not have 4-ohm speakers now, but an amp is a long term investment. There have also been a few cases of folks having problems getting all of the amp blades to work out of the box (I recall seeing posts by one MPS-1 owner who needed eleven blades to get seven that worked). You might be able to ask Emotiva about these - the MPS-1 has been in production for a while, and they may have sorted those issues out.