Is the r pronounced, or is it one of those disappearing letters that almost sounds like it's not there?
In some people's pronunciation there's a very subtle difference, but unless one does Slavic linguistics for a hobby, it doesn't really matter. Even speech therapists recognize those sounds as basically the same.
Here is some Slavic linguistics for you: "ż" is an etymological letter used in words where in other Slavic languages the sound is and has initially been [ʒ] (żona - жена), while "rz" evolved from what had been initially a palatalized "r", which is the case in languages like Ukrainian or Russian (rzepa - репа).
I've noticed the same thing when learning Polish (I'm a native Slovenian speaker). E.g. 'rzeka' would be 'reka' in Slovenian, and 'rzepa' would be 'repa', whereas 'żona' would be 'žena'. :)
More like, "rz" and eastern "r" both - evolved from the "rź" sound in older Slavic speech. In Polish, the "ź" fell into one sound with "r," in the Easern languages, "ź" fell into one with following sounds, making them soft. At least that's what I've heard.