That's a tough call. Honestly.
For me, I was a fan of Halo for years & only just recently started to play the GOW series. I am yet to play GOW2 or GOW3, however I do have them & intend to start playing them once I finish GOW1.
The game play is rather similar in a lot of ways, however I do feel like GOW does have more strategy behind it, which makes for a more challenging & interesting game all around. I guess if I had to choose though, I would have to say Halo just because I find more joy in playing Halo online with others than I do with playing GOW online. Idk if it has to do with the people who I've been playing with (good chance of that) or what have you... but, thus far that is my choice.
Ahhh... wait. Let me get more specific with my answer here. If we're talking online gaming then it's Halo, however... if it's a solo game & single player that I am rating then I would go with the GOW series.
Once again, back to my first line of this reply... that's a tough call. Honestly.