It's a good question, and a very valid one. Both of these are outstanding companies, but Loblaw has been able to leverage strong free cash flow generation to reinvest back into the business and really amplify margins. Over the last while here, Loblaw has doubled its operating margins which has resulted in larger free cash flow generation which has created a bit of a feedback loop.
Higher margins leads to higher free cash flow, which leads to more capital reinvested into the business, but also leads to more share buybacks which ultimately is a double-pronged way to boost earnings.
Metro is cheaper here, there is no doubt. I believe this is likely due to what you said. The adaptation of the rulebook, while Loblaw HAS the rulebook. Until any sort of disruption is witnessed or at least some market share being taken away, I'd expect Loblaw to trade at a Premium.
I tend to lean towards the one owning the playbook and the one executing in a big way, that being Loblaw. Management has been top notch and they've made all the right moves over the last 5 years or so here.