CPU is the only hardware difference between the lower 95x series:
- RUT950 - CPU: Qualcomm Atheros Wasp, MIPS 74Kc, 550 MHz.
- RUT951 - CPU: Mediatek MIPS 24KEc 580 MHz.
Higher-end models include an additional hardware change related to memory expansion:
- RUT955 - CPU: Qualcomm Atheros Wasp, MIPS 74Kc, 550 MHz and SD card support.
- RUT956 - CPU: Mediatek MIPS 24KEc 580 MHz.
Currently, RUT950 and RUT955 have more certifications than their alternatives and can be used more globally, however, eventually, RUT951 and RUT956 will catch up certification and approval-wise.
More about this device series certification here.
You can learn more about the RUT9xx series device differences.