Fuel wise, what you need to calculate is specific consumption in terms of fuel used / nautical mile. As 1 kt is 1 nm/hr, the equation would simply be Fuel burn rate/ground speed, which when multiplied with time, yields fuel burn/nautical mile. In your example:
3460pph/308kt = 11.23 lbs/nm
3824pph/371kt = 10.31 lbs/nm
So to go lower would be better in your case, looking only at this factor. The time saving is also beneficial in terms of maintenance and time related costs.