Can't decide which way of running feels easier. Is there a way to measure this?

I think the fact it doesn't affect your heart rate, makes me think it doesn't matter here. Go for whichever you enjoy the most.

If one makes you feel more beat up at the end of it, do the other (unless you are trying to change form or something, then ok for a short while, but easy should feel easy at the end of it).

Mixing it about imo is good as well.

HR is the best indicator here, so both seem equivalent on running economy.

There is usually one way of running that comes to you most naturally, and this form is not rigidly fixed, rather it dynamically varies slightly over the course of even a single run as well as across runs, so it’s best to do what seems intuitive as at the very least, it’s mentally less stressful to have to think about it and perk up.

Unrelated, most of the form mumbojumbo is indeed rubbish, and the fact that your form improved because of tweaking it doesn’t invalidate that as 1) you have no way of disambiguating how much of your form change happened naturally simply because of the body’s natural tendency to optimize economy over time for any related activity; and 2) you have no way of knowing that your economy wouldn’t have improved just as much or even more if you gave your body time to find and optimize its natural form that could’ve been different from your current one.

This is interesting. I think I need to commit to some longer runs at the same "technique" and compare HR data to really find out. I think I'm sensitive to doing the correct thing because I'm building back up after a long injury and want to get it right.

In response to your unrelated paragraph, I don't know. I agree with some things. There is indeed lots of bollocks in the form mumbo jumbo stuff out there. However, as a music teacher by trade, I believe I have a huge amount of experience on training "technique". And for activities both "natural" and "unatural". I think many people believe that just because as a species we run about the place that our "habitual" technique is correct or at least, we shouldn't interfere with it and it will optimise over time. I don't agree. At least, not entirely. It will optimise to degree over time but, there's a chance it plateaus at one point without conscious intervention OR one can save loads of time by consciously applying some sound principles early on.

I teach guitar and make sure I teach things that I believe facilitate the playing of more complex music down the line and produce the best sound and allow the person to practice for the longest amount of time (and help prevent overuse injuries if they ever get up to dozens of hours a month of playing). When I think back to being self taught before I had lessons, I had bad habits that I might not have fixed for years. I've met professional musicians who are really help back by an aspect of technique or who get pain or injury.

You may say, yes, but this is not a particularly "natural" activity. To which I'd reply, all of the above can be applied to vocal technique and that is definitely a natural activity. We start doing that before we can run. Some people sing and train for years without being able to accomplish what they want until they consciously change their "natural technique".

Another comparison you might make is speech therapy. Most kids learn to speak perfectly fine. Some don't, even though speaking is a 100% natural activity that our bodies are well adapted for, and they require expert intervention and special drills. If they don't get speech therapy, they'll partially approximate some sounds but never master them fully.

There are probably some runners who are the equivalent of kids who need speech therapy. Their bodies could be more efficient at running if they did some specialized exercises. The problem is that there are a lot of different paths to running form based on individual physiological differences, and a lot of the people dispensing advice about running form are amateurs. There may be more people peddling voodoo than actual experts when it comes to running form.

muscle oxygen saturation (sm02) using a wearable near-infrared spectroscopy

you can monitor oxygen utilization at the leg muscle level in real time to compare the rate of change and/or before after values

Hobbyjogger problems.