I think for most people I would lean towards recommending the Shaptons over the Naniwa SS, particularly if you are able to find them at decent prices in Tokyo. The Naniwa SS cut fast (not as fast as Shapton) and are even wearing, but they are quite soft and can gouge easily if your technique is not steady, but I guess on the plus side it could be seen as an aide to help give more feedback to improve your technique. They also tend to clog and build up compared to Shapton.
That said the Naniwa SS do an effective job, and the fine and higher grit stones achieve an excellent level of polish and very keen edge, hence they are quite popular with the straight razor crowd. But it is fine if you want to try both, but in that case I would go with your possible idea of having the Shapton 2K and Naniwa SS 8K, since Naniwa SS best selling point is in the higher grits. It is a somewhat large jump to go from the 2K to the 8K, but I imagine that progression would be acceptably OK. Ideally though if you had a 5K, for some knives you could just decide to stop there or use it to bridge between the 2K and 8K. Because the Naniwa SS will gouge easily and also builds up quickly, it is important to have a lapping plate and optionally a nagura stone (not included) for cleaning when it clogs...well in general a lapping plate is a good thing to have to keep your stones flat, but it's more urgent for the Naniwa SS not because they dish easily, but because you'll need to smooth out the occasional gouge (though hopefully over time this becomes rather infrequent as your technique improves).
A few more things of note regarding the Naniwa:
First and foremost to be aware of for 8000 grit is that there is the Jyunpaku/snow white version and the regular superstone version that is yellow. I have not used either, so you will have to do some separate research on that (unless you already had a particular one in mind).
There are 10 mm vs 20 mm thick versions, and you'll want to pay attention to that when comparing pricing. If cash is tight and the 10 mm version is being sold between 40-50% less than the 20 mm version, it may possibly not be a bad trade-off (depends how much use you plan to give it), and that would give you extra money to maybe pick up two stones for the price of a 20 mm stone (e.g. a 5k and a 8k or 10k....the higher grits last longer anyway). But if it is only a small difference, then going for the 10 mm makes no sense IMO.
There are also versions of the Naniwa superstone with and w/o plastic bases. I'm not so crazy about the plastic bases on the Naniwa. On the plus side having a base does provide some level of protection for the stone...especially as it gets thinner, as it's mounted and glued to something that is reinforcing it, but the plastic bases tend to slip on counters, so you have to put something non-slip underneath (or clamp it to a stone holder). I also worry a bit that if I accidentally drop the stone (or during transport) that a corner of the plastic base could break off, and then my stone can no longer lie flat on a surface, rendering it rather useless (unless I can figure out how to remove the stone from the base, which doesn't seem super easy at a glance as it's glued strongly and slightly nested). The wood base on the Shapton M15 are very solid and can't really be physically damaged, and they have larger rubber feet that more stable than the Naniwa. They are stack and store more easily and overall have a compact footprint. Well with Naniwa, I would probably still choose to buy the Naniwa with the base instead of baseless (especially the thinner 10 mm version...I'm not even sure that version comes w/o base), and I imagine in Japan the w/ base version is much more commonly found.
I don't know where to buy Naniwa in Tokyo, but I imagine they could be found in Kappabashi as they are a large stone maker.
As for Shapton M15, to me all signs indicate that they are either identical or nearly identical to the Shapton Profession series, just mounted to a nice solid wooden base instead of coming in a plastic case (which doubles as a so-so stone holder - andur's tip is good for when you have multiple Shapton Pros, you can put one on top of a case with a stone in it to stabilize it, but sadly that wasn't my case when I used one). Here's a
webpage description and picture of a Shapton M24 1000x (just a thicker version of the M15), and it comes in a nice box too. Personally I'd stay away from the M5 line though - only 5 mm of abrasive material, not a large cost saving, and it uses a plastic case/holder.
