Not long ago, I was on the market for a lapping plate as well.
The usual recommendations I saw (here and other forums) were DMT 8C (320 grit) and Atoma 400.
I settled for the latter and I'm very pleased with it.
Re: the Naniwa and Norton lapping stones, my understanding is that they're not as good, especially long term. With use, they may lose flatness and will need to be lapped themselves. I for one, decided to rule them out from the get go.