It seems that the 12-lead EKG is the gold standard for diagnosing many heart problems, but they're limited to fairly short samples (e.g. 30 minutes). Its possible to send a patient home for several days to be monitored with a 3-lead Holter Monitor, or a Zio patch, which clearly does better at detecting transient heart issues, but is limited by the number of leads (?).
Putting aside the additional differences in the monitoring period, what might you hope to be able to diagnose with a 12 lead EKG vs, the single or 3 lead ambulatory monitors?