The EcoPlus is a diaphragm pump and will put out the advertised 7 CFM without anything hooked up to it. It's a "maximum output" that is deceiving since adding a line, a diffuser, and dropping it in 8 foot of water puts some resistance on the pump and that resistance will reduce the actual air flow through the diffuser. You need to locate a pump curve for the EcoPlus 7 to determine what the pump is putting out in in your scenario.

The rocking piston pump is very robust and can create much higher pressures, hence maintaining a much more consistent air flow in deeper waters. The SC50 only loses 1 CFM from the water's surface down to 15 plus feet of water depth. The EcoPlus will surely give up producing air by the 10 foot level.