Well, buying one now means you will allready have an AmigaOne when OS 4 arrives. It's unpredictable to say but there is a chance that the AmigaOne's will sell that well, stock might run out and you will have to wait.
In the mean time you can build the system and test your hardware so you can immediatly use AmigaOS 4 and not have hardware problems which might mean sending your AmigaOne back. You might even learn a littlebit of Linux which becomes everso more important these days.
Also, buying an AmigaOne will mean you show you're support to the AmigaOS4/AmigaOne combo with some financial support.
Both the AmigaOS4/AmigaOne combo and the Pegasos/MorphOS combo are unfinished products. It's just whichone do YOU want to see as a finished product! I allready made my choice( check the sig). :-)