You should first decide which family you are going to use. For this you can choose depending on what you can find available and reading online about the features, advantages, disadvantages, etc of the families. As I personally do not use or program ARM controllers, I can not comment on this, but here are a few you can look at:
The earlier cores such as the ARM7 used a Von Neumann architecture whereas some of the newer cores, the cortex range at least, use a Harvard architecture, they did this to try to improve overall efficiency and performance without bumping up the clock speed.
The earlier cores such as the ARM7 used a Von Neumann architecture whereas some of the newer cores, the cortex range at least, use a Harvard architecture, they did this to try to improve overall efficiency and performance without bumping up the clock speed.