You might want to consider an Arduino M0 (pro) or MKR1000, giving you the advantage of a not too steep learning curve with the Arduino IDE + libraries and later on the nearly unlimited possibilities of Atmel Studio and real (JTAG) debuggers. Note that the MKR1000 does have a JTAG header, while the MKR zero for example does not. Both the M0 and M0 pro do have a JTAG header, but the Pro version has a USB debug port so you don't have to buy a J-Link or ST-Link.
It uses an Atmel/Microchip SAMD21 with an ARM M0+ based MCU.
If you don't fancy the Arduino ecosystem to start you might as well go for a Cypress PSOC4 (pioneer) board or an STM32F0 Nucleo. I personally like the Cypress IDE, but YMMV. Both feature a USB debugger, but the STM board has a ST-Link (= J-Link).