Visualisation program suggestion

[Wai-To Chan <waito %! at !% mountain.chem.yorku.ca>]

 Dear colleagues,
 		 I am writing this note to solicit your comments on
 any appropriate visualization package for developing molecular
 graphics applications on a SGI workstation. I am looking for
 a program that can extract from a cubic grid of
 various electron density properties (computed from Gaussian
 using the cube option) these features:
 2D and 3D contour plots
 3D multi-isovalued surfaces
 gradient vectors (in 2D and 3D space)
 Locations of maxima and minima
    Specifically the program must enable me to produce
 an isovalued electron-density surface.
 The program also must put out the cartesian coordinates
 of this surface which I will need for subsequent evaluation
 of other quantities on this surface.
   At present I have in my mind either AVS
 (advanced visualization system) or Iris explorer.
 Can someone comment on these two programs particularly?
 My primiary considerations are prices, user-friendliness,
 flexiblity and popularity among computational chemists.
    Any suggestion of freeware would be very useful
 although I've not been able to find one free program
 that meets all my needs.  Other than modular
 graphics package an easy-to-program graphics subroutine
 library (compatible with C if not FORTRAN)
 would also be helpful. Thanks.
 Wai-To Chan
 Department of Chemistry
 York University, Toronto
 Ontario, Canada
 email: wtchan %! at !% yorku.ca