You've pretty much hit the nail on the head. The stats clan rats have aren't good enough to justify twice the cost of slave particularly because they are both meant to be used as tarpits. Also you can shoot into a slave unit in combat as long as they don't include a character.
The only reason to take clan rats is typically to include weapon teams or as a character bunker.
That's the main reason why I love my 3 to 4 blocks of 100 skaven slaves at the front of my army they are dirt cheap, to bog down them harder enemy units, shoot into close combat (skaven style) & to also help shield my more harder units to come in on the flanks hehe.
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