I've had the Otterbox Defender (way too bulky) and the Commuter (bulky, but not offensive)...and just recently got the Spigen Tough Armor case via the "Challenge" bundle deal. Got the case & a pack of their best-in-the-business screen protectors for $20, which was a great deal IMO. After a weeks use, I can say the Spigen case is slimmer, lighter, fits better in the hand and is much more attractive than the Otterbox...but I can't help but doubt the Spigen would protect my iPhone in case of a real (5'-8') fall. I might be wrong, and I'm sure not going to on-purpose test it...but the Otterbox just feels more protective than the Spigen.
That being said...I still have the Spigen on my phone.