AT91SAM7X512 From ATMEL's website:
A member of the Atmel SAM7X series of microcontrollers based on the 32-bit ARM7TDMI processor. It operates at a maximum speed of 55 MHz and features 512KB of flash and 128KB of SRAM. The 512KB of flash are split in two banks of 256KB. The peripheral set includes a USB 2.0 full speed device port , Ethernet MAC 10/100 port, CAN 2.0A and 2.0B compliant controller, two USART, UART, two SPI, SSC, TWI (I2C), three 16-bit timers, RTT and 8x10-bit ADC. Typical core supply is 1.8V, I/Os are supplied at 1.8V or 3.3V and are 5V tolerant. An integrated voltage regulator allows single supply at 3.3V. The SAM7X512 is supplied in 100-lead LQFP and TFBGA packages.
STM32
There are various variants in the STM32 family; First pick one:
STM32 F1 Mainstream
STM32 L1 Ultra Low Power
STM32 F2 Hi-Performance
STM32 F4 Hi-Performance & DSP
STM32 F0 Entry-level
STM32 F3 Analog & DSP
STM32W Wireless
If you ask me it boils down to the question of which manufacturer's datasheets are easier to understand and for which one do you get a development / evaluation kit for free(or almost)?