Are they the same? I haven't found anything to tell me what makes them different of even how much water they flow. My assumption is the impeller is different, maybe the pitch of it, like a propeller, but no where have I seen anything on this. I want to buy a high volume one.

Heavy duty should have bigger bearings, while high volume should have a bigger internal cavity and impeller for more flow. Since the drive pulley is an overhung load, better bearings might be a good choice, and flow is modulated by the thermostat so unless there is a reason, greater than design flow is probably not needed. That said, unless you have some way to compare the original design and the heavy duty, these names are probably just advertising.

I wish there was a way to know. So I guess heavy duty doesn't flow more just built more rugged?

That would be the implication.

If you have some particular units in mind, post up some links.

I replaced mine over the summer and had the same question. I put them side by side and didn't see a difference in the impeller. The price difference was only 10 bux more so I opted for the "high flow" model though I don't think there is a differenice unless it's in the internal cavity as the person above me mentioned