They serve very different purposes. Busycal is a calendar with todo's sort of tacked on (in the same way todo's are tacked onto iCal). Meanwhile Things is a task manager.
I would avoid Things. The developers have a very difficult time adding new features, don't seem to listen to requests from users, take their sweet time with updates, and it doesn't sync over the internet, only syncs over a local network. Oh and they communicate terribly.
There are better solutions, like OmniFocus.
I use iCal on my Mac, Calvetica on my iPhone and OmniFocus on both to manage everything. Tasks go into OmniFocus while scheduled events (work schedule, class schedule, appointments) all go into iCal/Calvetica.
OmniFocus syncs over MobileMe (among other options) and iCal syncs over MobileMe. I can sync anywhere.
With Things you're caught syncing at home otherwise your iPhone version is out of sync and may not have the latest updates.
Avoid Things.
Busycal is "ok" but it isn't much of a task manager. It's a great calendar for people with super crazy schedules. But it really needs work in terms of usability.
