As djmarchand stated above, it really depends on how many hours per year you are planning on running it. Northern Lights is truly the Cadillac of small gensets.
I had an 8kw unit I pulled out of a boat last year with over 30,000 hours on it, was starting to really push my luck... Other than routine maintenance, the only item ever replaced was an injector pump at approximately 20k HRS.
The replacement unit is a CAT, using a Kubota block, and I'll be happy to get half the hours I got out of the Northern Lights.
In my opinion (for whatever it's worth) if you are running the genset less than 150 hours per year the Nextgen should work out great for you, as will many others.
I did quite a bit of research before the replacement and would be happy to share the information with you. Just PM me if interested. Good luck in your search.