William and Louis have NI about half the time, but I also buy raw meat and blitzed raw veggies from a local source, feed them tinned sardines/pilchards once a week, chicken wings sometimes and mix some liver in with their other raw food once or twice a week. The reasons for not feeding NI all the time are partly down to cost and also to give them more variety inc. oily fish and offal every week.
I always buy the cheapest that NI have in stock, which is usually their Working Chicken or Working Turkey. The ingredients are only very slightly different from other NI products but it's cheaper as it's sold as a food for working dogs and is therefore VAT free.
William is 8 now, so I recently did some online research to try and find out if I should change his diet to a lower protein one, as that's often recommended by vets and dog food manufacturers. That recommendation is still out there, but there's also plenty of sources suggesting that it's not the % of protein that's crucial, but the quality of it. So (hopefully rightly) I'm not changing William's diet at the moment. He has 1 Yumove tablet every day, but that's the only "concession" to his middle age so far.
Personally I don't think there's much difference between the different NI foods and I can't see any real benefit to my dogs in buying the more expensive ones unless they're on offer.