A nice basic comparison here:
To me the DT user interface has been a comparatively confounding mess, and I found the devs to be fairly unhelpful in the several years I used DevonNote, then Devonthink, then Devonthink Pro. So I’m fairly relieved to have escaped that app orbit, even though I’m up to date on the DTP/Mac should I need it.
DT/Mac does win overall on features and depth, and if you need it you need it, period.
I have never used the iOS app but have read that there are sync limitations in the iOS app wrt file storage or editing, but I’m not clear on that at all. (People have complained about ‘jagged’ PDF annotation on DTP_TG.)
I do know that the tagging UI on Keep It seems to be nicer than DT. Otherwise I think I generally prefer the experience of EagleFiler/Notebooks on the Mac. (Presumably Keep It too, just by the look of it.) We discussed Notebooks more in this thread.
Curio markets itself as a giant whiteboard with modules that offer Calendar/Reminders/Evernote integration, while also integrating mind maps, lists, tables, index cards, notes etc etc – in a single whiteboard space. And there’s no iOS app. So I really don’t see much overlap between Curio and cross-platform apps like DT/KeepIt/Notebooks (or other Mac-only shoebox apps like EagleFiler). Curio is really its own thing, and I think its main competition these days is probably online (and therefore cross-platform) whiteboard products for creatives likeRealtimeBoard, ConceptBoard, Mural, Zeplin, IdeaFlip, etc