I agree with all of them. I think it also depends on what the final plan for the bike is...showroom original and rarely ridden, or some trackdays and fun, not too worried about future value(to a point).
If I was to buy one just "to have", it would likely be a minty 28SP.
What do I own, a fairly minty 21SP Pentax. Pretty rare by definition, but it stll sees the track and a few thousand kms a season. These bikes are extremely rare in this market, I have never seen any other than mine in person, and extreme mods will detract from the value. Most NSR's in this country are rarely ridden, let alone tuned, which is why the 28 seems to be worth more!!!
What I am trying to say is that the 21 is more easily enhanced with greater outcome at a lesser cost than the 28....maybe
Either way you go, know this, they are quite likely the most fun and smile inducing streetbike you will ever own!
Cheers and welcome!