I would probably go with the 5.1 card (SB0100) as well, if I was going to choose only one of them, but there is the ASIO mapping issue with 5.1 models, so if you do not want to deal with that, then use the CT4780.
What exactly is the issue with ASIO mapping?
My intended use is for composing, monitoring and play virtual instruments with a low latency as possible via stereo out. i don't need multichannel out and all mixing will be done within the digital domain. (mastering via WAVES bundle) I have no external mixer attached to this DAW just monitoring speakers. In this case sound quality would only affect the monitoring right?