Hey,
Speaking as someone who doesn't use a lot of missles, and doesn't really know the tech, I can tell you that a high tech plasma missle may be slightly better than a Capital missle V, but not much. If it is better, it will be better by being slightly quicker (higher speed to target). Unless you are a missle specialist, don't bother with plasma.
P.S. If you research ionic tech high enough, you will get engine damagine missles. Those are very cool, especially when combined with normal missles (point defense seems to target normal missles first, before specialty missles).
P.P.S I've had a few beer, so please forgive me if I give you bad advice. Personally, I find missles to be very effective early on, but to be gradually beaten by some of the high tech beam techs, especially Phased beams. But don't listen to me, listen to the elite players who have much more experience.
As an relative newbie, though, I can tell you that I never use high level missles myself. For what its worth
[This message has been edited by Mad_Lear (edited 21 August 2001).]