I'd say the Teensy is in the Arduino++ camp there. It uses the same libraries, etc, just has more power processing, a pretty nice price, and some interface bits and pieces that are very nice, like being able to act as a usb device and host. A lot of hardware, I'd argue is better to be treated as a library -- there are libraries and hardware that have definite feature sets, some of which overlap, that allows you to choose the best tool for the job.