Good or Bad Idea?

I was painting one of my Terminators from Space Hulk last week as a test model for my Grey Knights I plan to pick up when they are released.  After I was done, I realized how much my skills have improved since I painted my Marines many years ago.  Aside from the fact that GW doesn't make the primary color I used in them (Midnight Blue), I think the work I did with Sergent Giddeon was much better and more interesting looking.  It is basically a Deathwing look with Necron Abyss as a secondary color.  My dillema is do I want to repaint ALL 5000 points of my Marine army, or just the Terminators?  I'm leaning towards the Termies only for now, but I might do them all this way eventually.  I guess the Grey Knights will have to remain Grey for the time being.