I purchased both the Spigen Slim Armor and Incipio DualPro.
Both are a 2 piece case with essentially the same design around it. What I found... 1. Spigen's TPU inner material is much harder and not as flexible. The buttons are harder to push and harder to control (want to raise the volume up 1 level but you accidentally hit it 5 levels etc)
Sigen's outshell is glossy so it's a finger print magnet. It has a really slim profile however (slightly slimmer than Incipio).
Incipio has a slightly higher "lip" than the Spigen. Meaning when I set the phone face down I don't worry about scratching the screen as much.
Incipio has a matte finish and is much more grip than the Spigen
The Incipio is also $15 on Amazon http://www.amazon.com/Incipio-SA-375-DualPro-Samsung-Galaxy/dp/B00BRI4XR0/ref=sr_1_1?s=wireless&ie=UTF8&qid=1371916438&sr=1-1&keywords=incipio+dualpro+galaxy+s4
I have this one. Had it for about 2 weeks now, no problems. http://www.amazon.com/Incipio-SA-375-DualPro-Samsung-Galaxy/dp/B00BRI4XR0/ref=sr_1_1?ie=UTF8&qid=1394810588&sr=8-1&keywords=samsung+s4+case+incipio