Hi Old Horse,
Unless you plan to go longboard racing, I do not understand why you
would want such a huge board.
I think the Rio will be much better for you as it combines many of the features
of the longboard with many features of a shortboard (when you kick the centerboard
up) so you get a blend of the best of both types of boards.
The goal of most entry level windsurfers is to get onto a planing shortboard at some point.
With this in mind, if the wind comes up to 12 knots, the Rio M or S will become a large shortboard (that's what most impressed me about the '08 Rio M that I have).
The Rio (even in TufSkin) is nearly 1 Kg. lighter in weight.
The adjustable mast track on the Phantom is a must for racing, but simply something
else to get jammed with sand or corrosion at your skill level.
The big 78 cm centerboard is nice, for racing, but other than a few degrees of upwind angle when sailed by a skillful and experienced longboard racer, your upwind angle would be similar on the (within 5 deg.) Rio M or S.
Hope this helps,