Flat vs Hill Time Trial

Which is harder-a hillclimb or all out on the flats? I did both today; starting with Old La Honda hill climb (21 mins) and then 7 miles on Canada road (20 mins). I did this without aero equipment, and yes, did the climb first. Difficulty was judged by heart rate output.

Check out the graph-
- With the uphill tt, avg'd 169 bpm and progressively rode harder (hitting 180 hr)
- with the "flat" tt, first half as much higher heart rate (avg 176) and hit higher (185)
- at the turnaround of the flat tt, I was spent and had to struggle back, avaraging 164 bpm.
- overall, on the flat, avg'd 170 but the effort was not maintainable.

I think the hill truly helps do the work. New found respect for (flat) time trialers.
Either that or I don't know how to ride flat TT's--where maybe one needs to save power for the return.