I'm already using RightCapital for planning, so the appeal of having tasks built right into the platform is nice, but like that Knudge can send reminders leading up to a task due date.
Otherwise, they seem pretty similar though Knudge does seem to have a little more customization and can also be sent via SMS.
I would use these for client action items coming out of meetings, informational reminders, meeting scheduling reminders with link to Calendly, etc. I'm sure there are tons of other use cases
Welcome any thoughts or feedback...