No the best metric al hence why I started with
Hard to gauge intensity from the info provided
I was trying to get across that an typical road ride and a typical MTB ride for most people with the same average HR for both should feel easier on the road due to the more constant and controlled effort rather than constant spikes between very high and very low effort typical of MTB. This seems to be an opinion shared by a lot of the other comments here. Of course it isn't set in stone, but is a general observation making me feel the road rides tend to feel easier.
If someone is happy riding a MTB all day then a road ride between 137-157 bpm shouldn't be too taxing on the legs but pretty boring unless you have some cracking scenery. Granted HR isn't a great measure of intensity or how the total energy was used over the ride but 157bpm max over the ride shouldn't dig too deep for most people. As the OP has said, the aim was a Z2 ride which should be pretty comfortable.
Nailing a road ride at a constantly higher intensity than is sensible makes a road feel harder for me simply because after plodding through mud at pretty slow speeds the far greater speed is addictive and I can't keep the excitement in check Another feeling shared by others here.
Of course everyone's experiences and local riding vary which is all interesting in itself.
OP, I'd imagine a slightly different position on the road bike accounts for a lot of the initial added difficulties. Again, generally speaking, the two tend to use slightly different position for most people despite best efforts to keep everything the same, using slightly different muscles in different proportions.