In short: "Jawdropping!"
The long story:
1) You'll gain a window-like viewing experience thanks to its 72-degree AFOV (OK, Nikon says it's 64.3 degrees, which is still a lot more than the zoom's).
Advantages of the greater FOV are easier target finding which will provide more time for IDing the birds and a better overview of the scene since the "porthole" view is gone.
Last but not least, the aesthetical merits of the image increases significantly. The dark area outside the field is so far out from the image that you'll even sometimes be unaware of it. This also increases perceived brightness a bit.
Much better eye relief: If you use spectacles, you can keep them on your nose with the Wide MC. Not so with the zoom.
This may not sound as much to brag about, but from what I've read, very few will favor the zoom after trying a Fieldscope "Wide" eyepiece.
//L