Ok I purchased both cases and figured I would choose between the 2. First let me start by saying that I usually am not a case guy and usually prefer my phones naked. I feel cases just add bulk and since I carry in my pocket this is my main concern. Anyway as far as straight protection goes the Otterbox is better in my opinion. The Seidio however is definately smaller and less bulky. The finish and feel is personal preferance I guess. Seidio is rubbery and does not slide on smooth surfaces and does feel better in the hand. The otterbox is smooth and sleek and has a plastic feel which makes it easier to remove from the pocket.
The long and short of it is I feel both cases take away from the cosmetics of the phone and both make the phone feel quite a bit larger and more bulky. I will probably be going naked yet again with juts my screen protector.