Regarding your question on I2C, there are now two alternative “unofficial” ways to develop for the cortex, both use the eclipse IDE as the front end. I am the author of the ConVEX firmware, this is an open source alternative firmware for advanced users trying to do more with the cortex than the educationally biased development systems offer. I will be showing how to interface a third party I2C sensor to this in the next few days, it really is quite easy.
ConVEX on the cortex may be a viable alternative to going with an arduino, the cortex hardware is certainly more capable and user friendly.
Most classic arduinos are based on the ATmega328 running at perhaps 8MHz or 16MHz. There is now one with an arm processor but it’s fairly new. cortex IO ports are ESD and short circuit protected, two motor drivers are built in, USB for WiFI is available without stacking up more hardware. I guess I was really just comparing the 72MHz STM32 with all it’s functionality with the ATmega328 that’s on most of the cheaper arduinos.