Go with the Northern Lights Generator. You won't regret it. In your gut you know Beta is second choice.
Our NL is a 753 is an 8kw built in 2001. We just bought the boat last year. I decided to go through everything for PM and was pleasantly surprised.
The raw water cooling coil is designed for easy access even when it's been neglected.
All the service items are on one side for easy access
The exhaust flange is cast stainless steel. I replaced mine sine it was 14 years old, but it was still in good shape.
And after 1500 hours of running the generator coil enclosure was clean, without black dust or signs or wear.
The factory support is unmatched. So is NL Parts and Technical help. When you call for assistance and they give you the part number, they will also give you the retail price so a dealer cannot rape you. Try that with Yanmar. They will never tell you the retail price and so their dealers F#*k you every time.
Hind sight will confirm NL was the right choice.