I'm gonna go out on a limb here and say I think your "battery dude" has only dealt with optimas and has very little education regarding other AGM batteries like Odyssey, Stinger, XS Power, or Kinetik.
Manufacturing companies have used AGM batteries in tractors, forklifts and other heavy duty equipment for many years. They do because AGMs are more reliable and need no maintenance.
I believe I would rather have one AGM than two normal batteries.
I have yellow top optimas personally. They work but I doubt I'll buy them again because there are better options available.
I despise red tops.