We went with dedicated NS nodes in London, San Fran, Bangalore.
We started off with 1 core+1G ram per node but as they do carry docker and other things they became rather CPU-bound during updates, 2 core +2G ram plays much nicer.
I'm not convinced about the Bangalore node though, the network is not as stable and I cannot stop users on the other side of the world from hitting upon it, fortunately for most TTFB is sill a much larger consideration than TTDNS.