they're entirely different beasts. dd-wrt is more or less a normal linux distro that targets embedded devices. it has a "unified config" that abstracts away configs for many packages, but those abstractions are leaky. when you log in by ssh, you're using a shell and editor to edit config files.

compared with RouterOS where the Linux bits are almost completely hidden behind the proprietary config engine (and the proprietary cli and webuis that power that config engine). for the most part with routeros, you'd be hard pressed to know that you're even on a unix let alone linux.