When i was searching, the Phasor (an 1800 rpm generator, same as NL) was about $1200 less expensive than similar NL. But here are some specifics why I ended up chosing NL.
Here's what 50+ years of R&D gets you. Matinization. Yea, the engine seems off-the-shelf, but a company the size of NL can work with the engine (or Gen head) OEM to make specific design tweaks. Other companies such as NexGen have had to retool their designs when an engine manufacturer made a change (several years ago, NexGen had to adapt their raw water cooling system due to change in PTO output configuration by Kubota).
NL enjoys an extremely intimate relationship with their customers and adapt to recurring issues. They have changed alloys in exhaust elbows when they didn't last as long as needed, changed main seals, etc. As a result, NL has an extremely loyal following with some serial boat owners having purchased a dozen generators or more over the years.
The research NL puts into their marinization is second to none. One of the videos I posted above discusses how NL tweaked the design of the exhaust to dramatically reduce noise. Other manufacturers talk about sound shields and gen-seps. NL goes further.
Global parts and support. We've all heard people on forums say "Kubota is great - walk into any tractor supply place and get the same part for 50% of the same at a marine store." Except rod bearings, valve guides, and piston rings are not what normally breaks. What does break are the bolt-on items: Relays, sensors, pumps, belts, seals, exhaust elbows (as Sunchaser notes), heat exchangers are what give out long before the engine or powerhead. Good luck finding them at a farm store. You will need customer support. NL has a global network and a proven track record of topnotch response.
Northern Lights has a YouTube channel which is understated and borders on awful. I swear, if these guys ran a sushi restaurant, they would market their food as "raw, dead fish." But I'd encourage anyone thinking about a new generator to spend some time watching their videos.
Peter