Whatever the Boss CS pedals do. Sustain is what I'm after. As I understand it, the pedal is just a compressor with the sustain knob controlling several of it's parameters as a 1-knob. I can get it with my plugins, but I'd like my old CS-2 pedal for live work (jams and possible stage). So I suppose it would be your second choice.
I never paid any attention to the compression, per se. I was only concerned with the sustain effect.