[policarpo]

Honestly there would only be a few valid reasons for switching in my opinion:
. Cloud Syncing (it just works)
. Context View (sometimes I like reviewing by contexts instead of Projects and Areas)
. Customizable Fonts
. Better feature parity across the product line
. Multi Task & Project view within the app (you need to Print to see this in Things)

Again these are just my opinions. I really like Things and hope their Cloud and Collaboration tools do what I hope they do.

One great thing about both apps though is that if you have your GTD system down moving from one app to the other is pretty simple. Both apps are great but both have their unique issues.

Cheers,
-policarpo | GTD2CYA