The IG wins, imo.
True/Peak Legion has displayed a greater scope of power, but I believe the IG's depth of power is still superior nonetheless -- and that is ultimately what matters most when we start discussing these quasi-omnipotent/Godlike types.
In a nutshell: I could see the IG(in its native universe or whatevs) overpowering/warping Legion without a whole lot of trouble. Conversely, I do not believe that Legion is capable of overpowering the IG to any extent. /shrug