I can't comment on the Line Prophet 90 specifically. But a friend of mine, who is an athletic intermediate, was on the Prophet Flites, and didn't like them -- he found them difficult to turn, and instead preferred a previous rental (don't know what brand/model) that he described as "carvier." Consistent with that, I've been on the Prophet 98's and found them a bit ponderous until you got them up to speed -- they really have more of a GS feel. Now, the Prophet 90's might be an entirely different ski; but, as an intermediate, you want to learn to learn to turn the skis with pressure and angulation, rather than having to muscle them around. And if the Prophet 90's share the characteristics of the Flite or Prophet 98, they might make the former more of a challenge than you want.
It's great that you're demoing -- it should be readily apparent when you're on the skis whether this is an issue for you.
Of course, you've got plenty of time -- your opening day is at least six months from now, right?