It is not that way today but for a long time the entry-level Cisco IOS router was PowerPC based, while the highend was invariably MIPS. On the other hand both architectural choices had nothing to do with the architecture and everything to do with choice of usable SoCs for that particular application (with Motorola/Freescale/NXP's "m68k Cisco router/Sun2 on a chip" SoCs being somewhat ironic in this regard).