I have the Missions in that size and a good friend has the 88's in 175 - I have skied them back to back same day (powder day Rusutsu, Hokkaido)and I don't think there'd be much in it weight wise - I was more intent on skiing than carrying them but if I had to bet on it I'd say my Scott's are lighter......... but then I would say that wouldn't I.
BTW to me the 88's felt a little more damped on the groomers than the Missions and both are amazing turners for their width. In the deep and dry though, the Missions are my pick. I just love them. I'd be very happy with a set of iM88's though - just don't ask me to swap 'em for the Missions.