Either one will suit.
You won't be able to sustain the maximum wattage for more than a few minutes, and if you can, your wattage meter is lying to you.
This is a link to an SRM (crank based power meter) power analysis from Mont Ventoux, one of the final stages of the Tour de France. The first part of the graph shows the "leadout" (Usually refers to the lead of a final sprint), which is where the domestiques (support riders) tried to crush it hard to eliminate anyone not ready for the climb. During this portion, they were averaging between 400-450 watts up a 6-7% grade, at about 30km per hour. The rest of the time they are around 350-375 watts.
When sprinting, you can hit 1000-1500 watts (Or at least the top pro sprinters can), but these are short burst segments, sustained no longer than 30 seconds at the very most.
Just as an n=1, I am usually right around an hour for the 40k, and I average 230ish watts, for about 3 w/kg. They average 5-6 w/kg. Your position on the bike and overall aerodynamics are going to matter much more than how much you can stomp on the pedals.