I've had a B3K for a long time, and at one point I decided it needed an upgrade to the B7K, so bought that as well.
I have stuck with the B3K on my board now, as I wasn't really using the B7K to EQ anything or get anything out of it that I didnt find possible with the B3K.
That's also the only thing that has stopped me having the new B7K Ultra on my board, but I'm still tempted in a way cos they look so nice!
I would be surprised if you ended up using a B3K and a Rat for similar sounds once you use a B3K I reckon, it has a kind of baked in sound all of its own, that some people love, some don't, but I very much do.
Is there nobody near you that has one you could try before you fork out the cash?