i think for a non coral tank, 4x39w would cost you more but if you upgrade it will be worth it, u might consider a retrokit that's only 2x39w or 2x24 if you are going to keep it FO for a while, then adding another retrokit later, whichever length you decide, with the 24" it will not be as good lighting in those last three inches and it won't cost much more to go with the 36" retrokit, it might look a little weird with the light hanging over though, but if you make your own hood/stand you could cover this up by making more room on the sides so you can't see the extra length, but i think pc lights of the same spectrum will produce about the same color, but in terms of future plans i think t-5's work out quite a bit better then pc's especially in terms of hard corals