I just finished Ghost of Sparta a couple days ago (coulda been over a week ago) and I wasn't disappointed. I liked the story pretty well and was a pretty nice package overall. My main complaints are 1) yeah the levels weren't varied: it was the same volcano area for a long while and 2) Deimos was there for like 15 minutes which leads me to my next point was that it kinda ended unexpectedly quickly in my opinion. I saved right before finding Deimos almost two weeks ago thinking that the best of the game was uphead only to pick it up a couple days ago and find the final boss battle. I wasn't expecting it to end so soon after finding him, I thought at least another half hour or another hour or so. Didn't they say it was 20% longer than Chains of Olympus (or something like that)? My gametime was certainly less than 6 hours while my chains gametime on the first playthrough was at least that long if not longer.
That aside, I think I favor gos over coo. To me both only really offered a couple of memorable environments, Elysium in CoO and Sparta in GoS. other than that nothing special. To me the story of CoO wasn't that great or memorable, all I remember was Persephone was ticked and something about morpheus helping her but thats about it. I think that GoS had a story that was more pertinent to the series. the graphics were better and the cutscenes at some points looked like the ps2 ones. If I had to choose only one to play of the two it'd be ghost of sparta.