The NST is good for hacking and it means you can then install apps on it like Moon+ Reader and Kindle app, just Google Nook Simple Touch hack and there are loads of articles. If hacking your reader appeals to you, there's no contest, get the Nook.
However the main benefits of hacking are all just reading related (it does make it more tablet like but a slow black & white tablet!) and since you use Calibre & Apprentice Alf, hacking the Nook isn't really going to bring you huge benefits.
The K4BNT is smaller and lighter (I found the NST very square and chunky but some people like that), and will probably just seem a lighter, neater version of your K3.
Spending a little more to get one of the Sony's T2 won't make the screen contrast any better, it might feel a little faster though. Or you could wait and see what the new basic Kindle will be like, as it too will feel a little faster than the older NST and K4.
With the front-lit screens there is a little bit less clarity in good daylight conditions but it's evened out by the fact the latest models (PW2, Voyage & Kobo Aura H2O) have Carta e-ink giving slightly better contrast than the Pearl e-ink the rest use. And in anything other than sunlight/good daylight conditions, having the light is a clear benefit.
Some people just don't like the front-lit screens though, and feel it makes the screen a little hazy as if the text is behind a pane of glass, and if you think they might bug you, ignore all of the good points and stick to your non-lit ereader, its your choice and there is nothing wrong with it.
