I actually went back and bought the Griffin. They both seem great and I thought I'd try out the Survivor. My biggest complaint about the mini is that it's almost too thin to hold easily. I also plan on tossing it in my bag with books and a laptop and want to have a case that gives me more peace of mind. These bulky cases solve both issues, making it much easier to hold as well as giving it great protection.
The Griffin does seem bulkier with thicker rubber and even thicker corners and 'handles' on the side. All the ports are sealed well including the rear camera (rear camera is exposed on the Defender). I think the Griffin is easier to hold since it isn't just a smooth, plain back. Rather, the thicker blocks give something large that's easier to grasp.
I do have a few complaints about the Griffin though, the rubber shell seems slightly too large for the plastic shell so it feels a little bit loose. I've had Otterbox Defenders on past phones and the rubber always loosens up over time and since the Griffin is a similar case, I would expect the rubber to loosen up over time but it already feels loose just a few days after buying it.
Also, the Griffin has a strange smell from the rubber part. I saw it mentioned about another case in a different thread and thought it sounded crazy but this case certainly has some sort of smell coming off.
I'm still debating about whether or not to keep the Griffin. I might exchange it for the Otterbox and see if its any better. This post ended up a lot longer than I thought lol