A:Answer Allegedly, the Megablast is louder than the Megaboom 3. It certainly has no increased sound fidelity, and the loudness comparison is best left to individual preference - how often are you expecting to crank the unit to max volume, anyway?
The big difference seems to be the presence of Amazon Alexa voice assistant capability on the Megablast - as a built-in feature, it essentially means you’re buying an Amazon Echo with superior sound quality (and portability, of course).
If you’re deciding between these models and plan on using them as streaming speakers first, I’d recommend sticking with the Megaboom 3. I think the reduced price tag more than makes up for no voice assistant.