When comparing Simpletasks Dropbox vs Toodledo, the Slant community recommends Toodledo for most people. In the question“What are the best GTD apps?” Toodledo is ranked 6th while Simpletasks Dropbox is ranked 19th. The most important reason people chose Toodledo is:
It's possible to create a to-do item by sending or forwarding an email.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro 100% Free
Just like todo.txt cli, this is 100% free (although donations are accepted).
Pro Also comes in a cloudless an Owncloud version
For those who don't care about syncing there is a cloudless version while people who run Owncloud may opt for the Owncloud version.
Pro There is an add to calendar button
This not only tells one's calendar (and thus any devices that sync with your calendar) about the todo, but allows all synced calendars to play an audible alarm when the todo is scheduled to be finished.
Pro Seemless compatibility with todo.txt cli
Let's one sort to-dos by due date then priority

Pro Can create to-do items via email
It's possible to create a to-do item by sending or forwarding an email.
Pro Decent filtering and sorting
Pro Priority organizing
Pro Great GTD implementation
The GTD principles are very perfectly supported by Toodledo.
Pro Functionality-rich
Integrated sharing, note taking, habit tracking, and outlining make this a more powerful tool than a simple list of to-dos.
Pro Upload From CSV
Quickly upload an excel (CSV) file template of tasks to a folder for complex but repeating projects.
Pro Well documented, free-to-use JSON API
If you adopt Toodledo as your productivity system, you can be confident that you can extend it to fit your needs with this programmatic access.
Pro Can tag items
Pro Space to add lengthy text
Notes area that can then be hidden.

Pro Sends a daily email reminder
Toodledo reminds you via email of the tasks on your plate for the day.
Pro Apple Watch support
Integrated support for Apple Watch lets you check on your to-dos on the run.
Cons
Con Must use DropBox service
Con As far as I know this app is Android only
I believe Simpletasks is an Android only app.
Con Free version limited to 20 folders
Con Android app has annoying notifications
Whenever the app fails a background sync action for any reason (server issues, lack of internet connection, etc.), a notification appears.
Con Good as a starting point
Toodledo is a good starting point, but is essentially the same as an Excel spreadsheet where you have to build your workflow via the saved searches.
