IG can squadron their tanks, AdMech cannot, so a tank-heavy IG player can fit more tanks in a single detachment. Russes are also T8, while both AdMech tanks are T7, which makes AdMech more vulnerable to IG plasma than IG is to AdMech plasma (though AdMech plasma is much less likely to kill itself from overcharging, since they have a lot of mitigating stratagems).
IG also has more transport options, some of which get guns much nicer than Cognis Heavy Stubbers. Flyer quality is also in question, since we don't really know what each flavor of the Archaeopter is going to be fitted with, though it's more efficient model-wise, since all three versions come out of the same box.
Finally, IG basic infantry is cheaper than Skitarii. IG Infantry is 4 points per model, IG Veterans 5 ppm, and Militarum Tempestus 7 ppm, while Skitarii Rangers start at 7 ppm, and Vanguard at 8 ppm.