Due to the higher content of milk solids yoghurt is slower to digest than milk - it tends to 'stick' in the stomach more (it co-agulates a little). This will especially be the case if it is one of the those 'diet' type yoghurts with added pectin, inulin or polydextrose type things in it.
Also, if it is an all natural plain yoghurt (as in, if it still has live cultures in it) then, due to the bacteria present, the carb content will be a lot lower than what is stated on the label - because the bacteria cleave the lactose into galactose and glucose and the glucose is used by the bacteria for energy (turned into lactic acid, which is what gives the yoghurt a bitter taste). So you essentially do not know how much carb you are getting.
And - Due to this alteration in glucose/carb content it will also have a lower II than milk.
So although I use natural, fat-free, sugar-free plain yoghurt pre-workout I stick to milk PWO.