HP is what matters. But it has to be usable HP. Peak HP is meaningless if it happens at an RPM that can't be used for your given speed. The "new" V10 makes less peak hp, but more horespower away from the peak, which makes it more usable. That, and the 6-speed makes more available too.
Finally.... I strongly suspect part of the difference is how they measure HP. Ford can "down-grade" the HP rating by selecting an RPM where all the EPA requirements are met. The "new" V10 has to meet the new EPA green house gas restrictions for medium duty trucks.
From all reports, the 320 hp V10 actually performs better than the 365 version. Also, in the F650 the 320hp version got a bump to 37,000 pounds GCWR from the 33,000 that the 365 version was rated for.