this is gonna be a rather opinionated response, but...
I've messed with a lot of my friends iphones, all different kinds of cases out there....
My friend had the Otterbox Defender. It is the most solid case I had my hands on, it was also one of the ugliest, biggest cases as well. I personally would never get one.
My other buddy has a Speck Candy Shell, and it was beautiful, still a tiny bit bulky but, overall not bad. Seems easy to pull it off and put back on (for those who want to dock their iphones or put them on windshield mounts, etc)
If I was to choose over those two you suggested, I would go for the Speck.
Frankly I'm going to probably go the cheap route (cuz im cheap

), and get one of those Gel cases (NOT silicone) for 5 bucks on eBay. I have one for my ipod touch, and for only 6 bucks (included screen protector) it is DYNOMITE! Easily slips off when u need it to, but wont accidentally slip off ever.
All I basically have to say is, you don't need to throw a lot of money in for some worthy protection. But, if you have the money, it couldn't hurt
