Congratulations! I only just asked to be added... guess I have 4-5 years ahead of me! I'm pretty sure mine will be a 5-string Mk3, coolest walnut facings we can find, fretted rosewood board.
I've only played one Wal and it's the one I'm looking at right now. It's got a fretted ebony board, and holy s**t does it not sound like rosewood. This borders on a religious argument, but I am very much in the "fretboard wood is a REALLY BIG deal" camp.
That said, my thoughts aren't based on having a Wal with a rosewood fingerboard in my hands, but on paying attention to differences in how different wood sounds on all sorts of different basses over time. Take that how you will...
I think rosewood is the default for fretted Wals for good reason. It sounds great, and it's... more forgiving. Warmer, maybe even a bit more growley depending on what that means to you. Think more woodey growl, not clankey growl.
Ebony is a bit more extreme. It's REALLY responsive... you're going to get every subtlety you put in right back out, and if you're sloppy it's less forgiving than rosewood. Tight, great note definition throughout the harmonic range, really cool top end "springy snap" that's not at all like what maple does. It sort of "compresses" when you attack it really hard. Think Marathon or Territories from A Show of Hands. You are hearing an ebony fingerboard... in contrast, think Middletown Dreams on the album -- second verse when he digs in. That's a rosewood board on Peter Collins' bass. It's not the same sound.
I'm not saying one is better... which is why I want both in my long-term arsenal. 4-string fretted ebony for the "hard" stuff, 5-string fretted rosewood for everything else. 4-string fretless ebony, and I'll need a fretted Pro IIe. That should do it.
I know one or two Wals with maple fingerboards have leaked out over the years (a certain Lefty...). Apparently Paul won't do it. I would be fascinated to hear why... I have some guesses...
The way I visualize this is when you pluck the string, unless you're playing way up high, the piece of wood the string is interacting with the most is the fingerboard. Ebony is like shooting 70mm filmstrip. It's really sharp, so you need to watch your focus. Rosewood is like 35mm. The colour is incredible. You can get away with a little more imprecision. Neither is the right answer for every project...
What sort of Wal sounds do you like?