STM32F303 is a M4 clocking at 72Mhz. It has strong ADC’s and build in op amps as well as 3 motor controllers on the larger chips. It is a very interesting MCU for specialised motor controllers, but have so far been priced in the 10++ USD region. Lately I noticed that CB and CC versions of the MCU are down to 3-4.- USD on AliExpress.
My interest in F303 is limited because I believe MCU’s like F405 and F427 are better choices, but F303 offer the LQFP48 package making it attractive on applications where a LQFP64 is to large. I have so far not considered it because of the cost. I will add veroboard friendly breakout boards for STM32 Cx chips later.
The Rx Version should be able to use my Rx Board. I have mentioned ST’s pin compatibility between MCU’s before. It simply means that if a feature is on both STM32F103CB and STMF303CB they will most likely be available on the same pins though registers and coding might be different. This makes it possible to start With a low end MCU and simply upgrade to get more Flash/SRAM/Speed or extra features.