Cases that are cut for specifically the iphone 4s or the CDMA version of the iphone 4 should fit the 4s.
Some iphone 4 cases will fit the original (gsm) version of the iphone and the CDMA version (the cut away around the mute button is large enough to support it in either position, basically). These should also be fine.
I've got an iphone 4 "bumper" from Apple that's been cut for both versions of the iphone 4, and it fits my 4s just fine.
As you are aware, everything is a trade-off. Smaller case typically means less protection. Bigger usually equals more, obviously.
With that in mind, I have used an Otterbox, for rough going, and really like it:
It seems to be a solid build, and, although the rubber casing has gotten somewhat more loose over time, Otterbox does have a good warranty.
Now, this next thing is no match for a pocket, and likely as big as the Survivor, but perhaps you would like it:
If you desire an even more minimalistic approach and would consider carrying the phone in a pocket you could always go with my all-time case favorite:
This setup doesn't have a belt clip, but the case feels very natural in your hand - so you don't lose the iPhone feel, but also don't give up too much protection. Note that the back of the case does scratch some, but most of that is aesthetics. And the screen protector is terrific.
Best Answer
The storage on a 4S is Flash-based, not magnetic-media-based, which would be my prime concern in a scenario like this—so you're safe there. I'm not aware of any other components in the 4S that would be affected by a magnetic case. I have personally carried my own 4S in various magnetic cases for about a year now with no ill effects.