The gen sep is a no brainer for reducing noise for 3 reasons:
Longer overall exhaust run. The longer the run the less noise when the exhaust finally exits the vessel
More torturous path. Along with the additional run most installations end up with a combined 360* of turns, each turn reduces noise.
Elimination of splash noise. Self explanatory and significant
Some high end gen seps incorporate a muffler/resonator section that further reduce noise.
Along with secondary isolators this is an important part of a quiet generator installation.