You said your friend is in pro audio, correct? To me that indicates a difference in priorities - specifically, if you’re recording or mastering a track, losing power is potentially catastrophic. If you’re listening to music at home for fun, losing power is an inconvenience generally.
Regarding his comment on sine wave distortion, unless I’m reading things wrong it seems like the Eaton is nothing special in this measurement? This is from their manual:
Output voltage distortion THDV% in normal mode:
a. Linear load: <2% linear load
b. Non-linear load: <5%
Here’s our measurements for output distortion:
Output Distortion (Resistive Load) <0.5% // 0.1% typical
Output Distortion (Reactive Load) <0.5% // 0.1% typical
At a glance I didn’t see any specs listed on the Eaton page regarding high frequency noise, so it’s tough to make a comparison there.
Not to say your friend’s experience is wrong or incorrect in anyway. If he has done his homework and he prefers using the Eaton 9PX, you won’t find me arguing with him on it. Personally, though, I see more downsides to a UPS than upsides.