I guess I have about as much experience buying used Guild dreads as anyone, so I'll strum on in...
As mentioned above, none of the different tonewoods is "better," just "different." If I had to order my Guild dreads on a scale of "bright and clear" to "rich and complex," I would order them:
Bright/clear... D-35 (flat mahogany B/S), D-44 (flat pearwood B/S), G-37 (arched maple B/S, I think the arched back on this one make a big difference), D-46 (flat ash B/S), D-50 (flat rosewood B/S) and D-55 (flat rosewood B/S)... rich/complex. The arched mahogany B/S D-25 is harder to fit into this scale... having both the brightness of mahogany but a strong dark bass....
They all have their distinct tone... so you do need to have an idea of what you like/want in terms of tone. If you like bright and clear, think maple or mahogany. If you like rich and complex, think of rosewood. If you want something in between, try an arched back D-25 or an ash D-46. Since most of my sales were online (eBay and other), I only tried two of my Guilds before buying... my original D-35 bought in Jan 1973 and my '76 D-50 bought in 2009. However, I was intent on trying all the tonewoods, so I didn't need to try them all ahead... the idea was that they WOULD sound different. I would have a hard time picking just one...
As for what's available in a range of $600-900... I'd say just about anything! Discounting the fact that my D-35 was paid for in 1973 dollars, only two of my Guilds cost me more than $1K... and they were just barely over (the 50 was ~$1030+tax, the 55 was ~$1125). My 46 was ~$950, but I paid more than average for the model, because I HAD TO HAVE IT (I had been watching it for more than a year on GBase, saw it sold, and then it appeared on eBay)! The others were less than $900... with the D-25 and G-37 ~$550 ea (none of mine are showpieces, tho' all except the 50 have cosmetic issues, from minor to moderate). IN most cases, rosewood will have the higher selling prices, but folks have found good rosewood dreads... DV-52s for example, for under $900. Even D-50s and 55s can be found i that range (usually with some cosmetic issues).
Anyway... tell us the sound you are looking for, and you'll get a hundred sugestions. IF you were in the area, I'd say come on over and try out the different woods.
Dave