I've got the S-type channel compressor, yes. I have to say, I've not used it for quite a while. There's nothing wrong with it, though. Indeed, it's pretty good for vocals, from memory. I must give it another whirl some time.
As for "choosing one thing over another", it's not really like that. I don't think X is better than Y or anything. Totally depends on the material. The vast majority of compression in my mixes is just the U-Type channel compressor and the CA-2A. Other compressors will come in if required.
I like using the bus compressor or channels if I want to get something deliberately squashed and pumping. I suppose more like a special effect than a compressor.