Mostly North American means you could do one of the US programs or AC. My opinion is look at where you are flying and how you want to get there.

If you are going to mostly be sitting on AC planes, and especially if "North America" includes some Canadian travel, you should look at staying with AC since you get some benefits aside from *G in terms of e-upgrades.

If "North America" means mostly the US then you can and should consider one of the US FFA programs for the reasons rankourabu outlines.

As others have noted some of the other overseas *A carriers programs may be able to get you *G easier but either they have some requirements for flying on their metal that you may not like, you'll almost never be able to use your status for upgrades and/or your awards redemption is sometimes limited. Plus if you fly enough to get E*50 on AC you've got *G every year anyway, which does make a difference when flying on partner airlines.