I picked up a Seidio Active case for the GS4 the other day. It fits nicely in the above holster, a bit more snug than the Speck CandyShell.
I still haven't found case Nirvana yet. I would like to try the Speck CandyShell Grip case, but don't want to pay $35 for it.
Honestly, I'm kinda leaning towards the Speck CandyShell and/or Grip as my fav thus far. It has really started to grow on me. I like the Seidio Active, but find the buttons are almost too sensitive, if such a thing is possible. I just wish the CandyShell weren't so slippery on the back and sides. Maybe the Grip will be the perfect option for me.