Welcome to FT.

It really doesn't make that big a difference whether you're NW or CO elite. On NW metal any NW or CO elite can reserve the exit row and have a pretty good shot at upgrading. On CO, there are more elites per flight and fewer FC seats, so upgrade chances are simply lower across the board, and they only let plats (both CO plats and NW plats) reserve exit rows in advance.

To me the biggest reason to choose one program over the other, though, is access to the elite phone line, since that's one benefit that's not reciprocal.