I had the same case struggle with the 3GS as well as the Iphone 4. Watching youtube videos from guys like Larry Greenberg helped some, but ultimately I had to buy the case and just use it for a few weeks. Everyone has such different tastes over many categories (like color, texture, size, style, protection, etc...) That makes it difficult to go from other people's opinions.
I like minimalistic designs, that offer minor drop protection, without adding too much bulk to the phone. I carry in front pocket so I need something that slides in and out easily.
I have tried:
Speck pixelskin HD
Otterbox defender (only use on special occasions for max protection)
Griffin elan form graphite
Magpul executive field
Speck candyshell (newest design with bigger camera opening)
My favorite right now is the candyshell.
Just watch a bunch of youtube videos and figure out what you like. Then purchase your top 3-5 and make sure you can return the ones you don't want. You will want to be able to compare them in person.