Classic Zone 2 VS ISM

So I’ve been doing a fair bit of ISM sub LT1 so far, which for me is keeping my heart rate under about 142 (max HR is 185). Are there any thoughts on how this compares to classic zone 2 (where I’d be having an upper HR of 125).

Presumably one progresses a bit faster with the ISM model?

25 hrs / week vs 10 hrs / week. With caveats.

What I’m saying in plain language: total duration is most of the difference.

The idea that you would progress faster in the 10 hr week scenario is the definition of SweetSpot training as a concept (as opposed to a training zone, which is how most ppl think of it…except, notably, Coggan himself)