These are almost opposite ends of the spectrum from eachother. The Defender offers more protection, but its an unattractive, bulky, silicone brick. The slider is sleek and offers less protection, but its also notorious for bubbling up screen protectors. I would recommend looking at other options from companies like Casemate, Speck, Switcheasy, and SGP. Figure out how much protection you need/want, and move from there.
Personally, I'm a fan of hybrid cases like the Switcheasy CapsuleRebel, Speck Candyshell, and to a lesser degree the SGP Neo and Casemate Pop. They offer plenty of drop protection without the brick dimensions. Most of them also have a lay on the table design + included screen protectors that wont bubble when applied properly. Otterbox has a similar case in the Commuter, but I really don't like the silicone they use so profusely in their designs.
http://www.switcheasy.com/products/CapsuleRebel_iPhone4/CapsuleRebel_iPhone4.php
or
http://www.speckproducts.com/iphone-case/candyshell-for-iphone-4.html
I have a black Rebel and I love it. I'm a klutz that always drops his phone in parking lots and bars. Yet, my white iP4 is pristine, still, thanks to this case. I almost got the Speck, but I liked the unique look of the Rebel, the slightly cheaper price, the accessories it comes with, and at the time the Candyshell had some issues that have since been addressed.