Both companies are well known companies that make great products. However, my personal favorite when it comes to minimalistic cases would definitely be the incipio Feather case. I use it on my 3GS right now and it is amazing! The matte-like finish on the back is a plus as well. But, you really can't go wrong with any of the cases you've mentioned either; trust me, they're well known brands when it comes to iPhone cases.
I have the white one and it looks amazing. Honestly, I would have to say almost any of their offered colors, save for that ugly green, would really complement a black or white iPhone. You can pick up the white unbranded version of the Feather from AT&T stores for $5; quite a bargain. Though, I will probably be ordering a red one from incipio soon...I've seen it in person and that color on a white iPhone i phenomenal.
Just know that the moshi case doesn't cover the Apple logo nor the top of the iPhone. The feather, on the other hand, covers both areas including the top around the buttons. I think that would be a better deal for you.
Ditto this. I have the pink one and love it, but noticed that the color is severely fading on 1 side, down low close to the black part. I'm assuming it must've been a defective one (hoping!), so I'm gonna run it back to the AT&T store this afternoon and hope that they'll replace it. It's only 11 days old, so I'm thinking they should.
Gorgeous case, though. I've gotten all compliments on it, even from men.