I just got the Otterbox case for the 8300 series. I've never had a Seidio case so I can't really give a good comparison of the two, except by going off the visuals of the Seidio cases. I would definitely say that the Otterbox will probably withstand more abuse, due to the larger casing and the inner membrane that covers the keypad, screen and the outer part of the trackball. On the downside, all this extra hardware also makes it bulkier and more awkward to use. The keys are harder to press and moving the trackball is a little weird, especially with the membrane on. I actually had to cut a larger hole out of the membrane to make the trackball usable. Of course the membrane is optional. You could always get another screen protector, or just cut the screen portion of the membrane off and just use that.
I would suggest that the type of case you get depends on your lifestyle. For normal everyday use, I would probably go with the Seidio case. For more extreme, outdoors usage (in the rain or snow, like maybe hiking, skiing/snowboarding, etc) I would recommend the Otterbox case. I'm really wishing I had gone with the Seidio case in retrospect, since I fall more in the former category, and I'm personally not really liking the added bulkiness of the Otterbox case.