You have to pay additional service gratuity on top of the NCL free at sea and the Wi-Fi package is very limited so they are not like for like packages. We have just come back from an NCL cruise and the basic Wi-Fi package was rubbish; much less favourable than the unlimited one we had with Princess last year.
You don't have to pay the service gratuity, it's optional, you can choose to remove it and tip as you go. Although most people will pay the gratuity, as it goes towards behind-the-scenes staff as well as those you see.
I agree that the included wi-fi is a bit rubbish, it's enough to log on to check and send emails etc, but not much more than that. That said, I'm quite happy to spend a week without wi-fi at all, we've always had minutes left at the end of the cruise, so poor wi-fi is not a deal breaker for me. Teenagers are a different matter though, and we upgraded on our last cruise so that our DD could facetime her boyfriend. The wi-fi was fantastic, but expensive - about £25 per day.
The best thing to do is decide on which ships/itineraries you like, then price it up, including all the things that are important to you. For us, that's a drinks package and access to a few speciality restaurants. We find that NCL gives the most bang for our buck, but it does, or course, depend what you are after. At the end of the day, you tend to get what you pay for, and you need to go with realistic expectations.