I have to say, compare with TDP, is risky, especially in AMD part
recall the last generation of both side: (3D load from Xbitlabs)
nVidia: G80GTX, G80GTS, G92GT, G92GTS, G94GT
AMD: R600XT, RV670XT(3870), RV670P(3850)
G80GTX: TDP 175W, 3D load: 132W
G80GTS: TDP 140W, 3D load: 105W
G92GT ...