I don't know much about Brittanys, so someone else will have to answer that part. I went with a Clumber not solely for rabbits, but because they seem like they can do many things. I do lots of different types of hunting - rabbits, grouse, chukar, quail, duck, goose, and big game. So I was looking for a dog that could do many things like track dead or wounded game, find and flush birds and rabbits, and retrieve the odd duck out of water. Plus, they are generally quite laid back in temperment (for a hunting dog anyway).
If all you will be hunting is rabbits, you might want a more "rabbit specific" dog like a Jack Russell, or mini dachhound. For me personally, though, there is very little on earth that is more irritating than a Jack Russel. For some reason I just can't stand the little buggers.
Not terribly familiar with clumbers, but Brittanys are mostly used on Pheasants. I know that niether one are much used on rabbits. They can be, but its not what they were meant for really. Clumbers are spaniels as are brittanys. Spaniels are bird dogs.
If you want a rabbit dog you need to look more into the hounds. Right now you are looking at sporting dogs. Look for Hounds and Terriers...they are your rabbit dogs. (though most any dog could be trained on rabbits...just easier if its in their blood)
Sage - Female American Kestrel 2008 -------------------------------------------- Zephyr - Female Red Tail 2008-2010 -------------------------------------------- Saffron - Female American Kestrel 2009-2010