I'll chip in with my reasons for going with a norton over naniwas for the main honing progression (i have a naniwa 12k). These are purely from the point of view when, having neither, i was about to buy.

Wasn't able to easily obtain naniwa combos in the uk (may not be the case where you are), I've heard that the naniwas dish a lot faster, i read some where that lapping them after every session or razorr wasn't unusual, and that they're really thirsty stones despite being advertised as not needing soaking. The naniwas we have here are the bench stones and are single grits whereas the norton combos came in a kit and where much cheaper.

Apparently the naniwas give a smoother finish and are faster cutters, but the nortons are a proven classic that everyone has owned. i figured that i wasn't going to be good enough for a long time to really appreciate or tak advantage of the differences in finish, that i would be better of getting a lower maintenance stone (that being said, i do tend to give my nortons a quick few circles to clean them off, most days before using them).

so in summary (and please note, purely IMO and having no prior experience with either)

The finish on the superstones is smoother, but the norton apparently isn't far behind
the nortons needed much less maintainence
nortons seemed to be the industry standard so to speak
a set of (single grit) superstones would have cost more

I find my nortons just grand, they do the job they were intended to do well, but am sure i would have been equally happy had i got a set of superstone combos.

I don't know about the lower grit superstones, but the 12k gives quite a lot of feedback, quite a "draggy" or "sticky" stone, where as the nortons are quite a light feedback, which i would say i prefer, but having no experience of the main progression superstones maybe saying "i've become accustomed to" is more accurate.