The obvious choice today would be armv7, using the R.Pi2. The only real advantage I see over x86 would be that the performance model of Cortex-A7 is much simpl

The obvious choice today would be armv7, using the R.Pi2. The only real advantage I see over x86 would be that the performance model of Cortex-A7 is much simpler than that of any modern x86 core, so it's much easier to isolate performance effects that one would like to study in an architecture course. A Cortex-M3 or -M4 would be significantly nicer still for that reason (also it has a nice stripped down ISA), but I don't know of any systems with as much nice tooling as the R.Pi.