The scene-a windswept hill in rural England, with a single water source.
The reason? Both choose to live on that hill but there is only one water source. Each group has an aerial scout and plenty of canny fighters on the ground, but which will emerge with the water hole and at what cost?
First off, I give the airspace to the Watership Down crew; I give Kehaar at least 15:1 on the odds that that clumsy crow manages to take him down. He's got the size, he's got the mother fuckin' meanness, and the only reason that he would get taken out of the fight is if the bitch ass clumsy crow gets lucky and tangles him in some pink string while they both plummet to earth. I think that regardless of what the NIMH rats are capable of planning, they will be doing it under constant aerial bombardment. Advantage, Watership Down.
In a close quarter underground fight, which is what I feel it would eventually come down to, if the rats were armed with swords and spears and possibly fire, they would have the advantage here. The rabbits are big and mean, and it would be dodgy, but if cavemen with fire hardened sticks can demolish herds of mammoths I'm sure rats with engineering skills and the viciousness and toughness of, well, rats, they could take them out by swarming one and panicking the others methodically. Bigwig is still going to take out some motherfuckers though, there is no doubt of that.
For leadership I'd give them even odds. The NIMH rats have a pretty human degree of sophistication compared to the rabbits, but Hazel is clever fuck in a society that venerates a clever fuck as the apex of the species and he has an honest-to-Frith psychic on his side. These are the things that make me think this will be a fight for the ages, and also that my brain is too profoundly affected by children's literature.
So who is it gonna be, folks? Who drinks water and who chokes on blood? I leave it to you to discuss. Feel free to ramble, argue minor plot points, and take a break for pizza and Jack Daniels. I'll wait.