[shadowgoose]

As a hobbyist embedded developer, I personally view the Arduino and Teensy as being suited for the same class of applications: doing one or two low-level things reliably and predictably, while serving as an abstraction layer to a general purpose computing platform, like a Raspberry Pi or other computer.

IMO, the Teensy boards have historically not only completely duplicated the functionality of most Arduino boards (including supporting the Arduino IDE), they've also provided additional features/peripherals not present on most Arduino boards. (Increased RAM, flash, CPU, peripherals like onboard CAN controllers, ability to enumerate as a variety of USB devices, and yes, more available pins)

I've also felt that that the Arduino boards are engineered (physically and feature-wise) to sell their branded daughtercards (Arduino "shields"). This is not necessarily a bad thing for beginner-level classroom environments. However, if you're looking to do something practical, it's a lot easier to throw a Teensy into a breadboard.