Without rats : curse + 4 cards.
With rats : curse + rats + 3 cards -> 4 cards, you trashed a curse and you gained a new rats.
You have to NEED these rats - who knows ? it can boost vineyards and remove that -1 point at least). So I'd agree with AdamH, but at the end of the day you go ratcatcher/rats to exterminate rats (at least it's nice thematically)
But just going for Ratcatcher and skipping Rats is better here. The only cases where that matters are if the game is ending and that one point matters (in which case Ratcatcher at all is probably wrong? That's not exactly clear) or you care about Rats being an Action card for some reason.
But if you care about Rats being an Action card, then why are you trashing it with Ratcatcher?
I've tried Rats with Scrying Pool and Vineyard. It's just not good. Just thinning is better, so you don't actually want the Rats.
And none of this takes into account the fact that there's a significant opportunity cost associated with buying Rats and then drawing Rats once you decide you're done playing them and they aren't already out of your deck. Ratcatcher is slow trashing and those Rats multiply super-fast.