Which is better: AVR or ARM?

The answer to this depends entirely on what you are trying to do. Generally speaking, AVR based systems will be less expensive and simpler to design and program. On the other hand, systems that require lots of memory and faster, more complex programs are likely to be better served with ARM based devices.

If you are just getting started, you should almost certainly should start with Arduino boards based on the simpler, more common AVR family. If you feel comfortable with Arduino circuits and programs, and need special features like wireless networking or complex mathematical calculations, then an ARM based Arduino is probably more suitable because the increased word size, speed, and amount of memory are better able to handle the larger and more complex programs.

Another reason you might choose an ARM based board is if you want to do any networking. With much more memory (256KB FLASH, compared to 32K on the Uno, and 32KB SRAM compared to 2K on the Uno, on all ARM based Arduino boards except the BLE and BLE Sense which have 1MB FLASH and 256KB SRAM) and much faster processing speed, these boards are ideal for handling wired or wireless networking protocols. As you’ll see below, most ARM based Arduino boards support one or more wireless networking protocol.