I think it's 6 of one or half dozen of the other. They are about a wash in the realm of AR's.
CMMG has been around since about 2002 or so, they got "big" about 2004. I have their .22 Conversion kit and it is well made and have had no issues with it.
Spikes has been around a while as well, maybe 6-8 years, I don't know for sure. They are big in the billet world of AR uppers & lowers, that is their meat and potatoes.
Apples to Apples, I would say the Spikes stuff is a little bit better, but not by a tremendous amount. I don't know the construction of the CMMG upper, but as long as it's good aluminum and not pot metal crap made in China, it is just slightly less "awesome" than a billet one from Spikes.
Good luck and let us know what you choose.