Ultimately the question boils down to is instant speed better than 50% more damage that Slagstorm can give
At least, as far as we know
It helps that Bloomburrow added many blue and black rats that care about the graveyard and threshold, so you can go that route, too
If youre looking for a budget repeatable option, I like the recent Vat of Bebirth quite a bit if you can enable it
The analyzer counts each from card text and flags exactly where youre short, so you know what to add
It might not seem like a lot since three mana isnt a huge commitment, but it can mean the difference between playing just one creature face-down and passing, and playing a creature face-down and then being able to flip it back over before your next turn