Yes i do believe that the i3 3rd gen U will be cooler. I'm just not convinced that it will be faster than the old i3 1st gen M since it runs at 2.5ghz compared to 1.9ghz. Is speed dependent mainly on the generation or the ghz?
Generally speaking, the average performance increase from 1st gen to 2nd gen is 12%. From 2nd gen to 3rd gen it is 6%. Therefore, going from 1st to 3rd gen is almost a 19% average performance increase. That means a 1.9GHz 3rd Core i3 is equivalent to a 3rd gen Core i3 running at 2.26GHz.