I stil haven't investigated all of SoundTrack's potential yet, but here's what I think is correct:
SoundTrack can record but I don't think there is any MIDI functionality whatsoever. Also, I don't know if you can record from external sequencers like CuBase and I sure as hell don't think you can sync to external sequencers.
As for Deck, it's a good app. I've been using it for like eight years now. Poor man's Pro Tools. I've never had the luxury of recording multiple channels at once (beyond two), so I never missed that bug.
I've been using CuBase for a couple of years and find it very annoying at times. SX is quite nice overall, but the low VST quality is getting old, especially now that I've compared it to SoundTrack's Audio Units plugins, which destroy their VST equivs. I'm very seriously switching to Logic for Audio Units as well as one more Apple app to integrate. I wish they had a cross-grade option, I'd do it in a second.