I did heart rate until putting my PowerTap back into a wheel at which point it was power.

Knowing more now, I go off the power I make at my aerobic threshold (aka AeT and VT1).

That's the hardest endurance pace you can have a conversation at, don't feel lactate in your legs, and aren't yet breathing rhythmically. It's the most power you can sustain for 4-5 hours without a significant rise in Rated Perceived Exertion. It's usually 25-40 BPM below your lactate threshold. If you measured lactate would be 2 mmol/L.

It varies somewhere between Coggan power Z1 and Z3 depending on how much you've been training your slow twitch fibers / oxidative energy system and where your lactate threshold is. Probably Z1 if you haven't been working to lift it; with traditional training my heart rates were all clumped together with a big difference between power and heart rate zones. Mid Z2 might be average.

Power is preferable because it doesn't lag or decouple like heart rate.