That's a hard call. I also wonder if the 600 will be too close to the 350. If I had just two wheels I'd probably want a coarser one for shaping tools and a finer one for sharpening. Between the 600 and the 80 I find I rarely even use my 220 wheel.
Do you use skew chisels and do delicate work with spindle gouges? If so, I would prefer the 1000.
Note that as Reed described in his CBN writeup on his web site, all CBN wheels are more aggressive when new then settle down after a "break in" period. That means you may see very little difference between how the 600 and 350 at first. Later there should be a bigger difference. Enough difference to make it worth while? I don't know.
In another post I see you mention scrapers. What kind of scrapers and for what kind of turning? I make heavy use of a variety small scrapers I grind from 1/4" or 3/8" stock and of larger NRSs with curves for face turning (all from Thompson steel), for example, these are some:
Attachment 373205 Attachment 373206
I sharpen all of these on the 8" 600 grit wheel on a bench grinder, then burnish a tiny burr with a carbide rod. The small ones are my go-to tools for smoothing and adding detail both on end grain and on the bottoms of bowls and platters. The larger scrapers with the curved edges are amazing on both the inside and outside of bowls and things. No power sanding needed - I usually sand by hand and if my hand is steady I can usually start with 400 or 320 grit. I mostly use scrapers for finishing cuts on bone dry and often hard wood - I rarely turn soft, green wood and don't hollow bowls with scrapers, although i will use scrapers to hollow boxes and other small things like goblets. I'm not sure the grit used would make much difference for hollowing, dry or green.
Do you know anyone near you with a 600 grit CBN who would let you try it and see? (I certainly would but I think it's a long drive from your house!) If you have two identical or similar tools you could compare the results of the two wheels.
JKJ