I'd suggest the Rio M for the following reasons (vs. Phantom 320).
The Phantom 320 is only 70 cm wide, so the Rio M will actuall plane
The Rio M (at 80.5 wide) is a very good compromise between a really
wide beginner board (like a Start) and the narrower race boards like the
The additional width = stability for beginners.
I've sailed the Rio M's for 3 years now, and spend alot of time on them with small trainer sails when teaching.
They go well in lighter winds, and have very good glide as the Rio M is a pretty efficient hull shape.
I have found the Rio M to be perhaps the best "transition" board ever made.
It has width (for stability) it has length ( for good glide in light winds with small sails)
it has the EVA deck (great for beginners and especially for kids); it has a good width
so that it planes fairly early and rides like a big shortboard with the daggerboard up.
It probably won't win any light wind races against the Phantom 380/320, but it will not be far behind.
As soon as the wind gets up to 12 knots, but a 7.5-8.5 m2 rig on the Rio M and you will be fully planing pretty easily.
Many seem to confuse the new Rio M's with the older Start based super wide Rio's .
Hope this helps,