AGM or Lead-Acid?

I have a Tuff Stuff brand Voltage Regulator(7545) and Alternator(8509RCDP). Until it gets to 1000RPM the voltage output is 12.3 to 12.5
At 900RPM it is 14.6 and at 1000RPM it is 15.1
Over 3 years I killed my AutoZone H7-AGM. This AGM normal sitting voltage should be 13.1-13.4 its only 12.3 and after it sits a few days it will not start the engine.
Testing with Tuff Stuff support they say the VR and ALT are working normally.

Since I need to replace my battery should I get another AGM or go with the cheaper Lead-Acid?
Does anyone have a preferred "Brand" that seems to last more than 3 years?
DKamp