Well Michael, I personally think that BRW will give you a sound that I heard Dan Bourgeois describe once as "lush." And I think that's true. My guess would be that you'd find the IRW a bit less so and probably more similar to BRW in the mid-range, but less "clear" in the treble and bass. I have found that when using Spruce soundboards on Hammered Dulcimers that BRW sounds better when used for rails, than either IRW, Morado, or maple(which is what I generally use). And that kinda' relates to the comment above. Maples are kind of acoustically neutral and the sound reflects through it much better than other woods, so when used as rails on HD's the "tone" of the soundboard is more important. On a guitar, maple backs and sides tend to let the soundboard do the work, which would help explain the better results with pick-ups.
Want the truth? I kinda' like the sound of mahogany best in most blind tests. I played two Larrivees at Roscoe awhile back and I picked the mahogany......but I love that look of rosewood. I've always preferred a D-18 to a D-28 too.......but don't tell anybody.