Compare Marriott lowest non refundable Vs Competing site lowest non refundable

OR

Compare Marriott lowest flexible Vs Competing site lowest flexible

OR Any of the above with breakfast (and perhaps other amenities?).

I think that if membership is public, free non restrictive then you could use other sites to compare but this must be always compared with the lowest rate when logged into Marriott, otherwise if you’re trying to circumvent thé freely available members’ rates then what’s the point in the Best Rate Guarantee?

My observations are also based mostly on SPG’s way so please correct me if Marriott is different.