I should of mentioned that you're going to die either way.
Well, this is a misleading topic title, then. Anyway, I pick Birkin. Because I'm going to die either way. So in his case I die in like 70 years of old age, because I outran the lumbering doofus and got away.
THE FIGHT WILL CONTINUE, AS LONG AS THE AMERICA INSIDE MY HEART IS STILL ALIVE
Nemmy only because I stand a chance of downing him to escape. Nemmy is far more vulnerable to small arms fire then Birkin ever was and while he is faster and more intellegent, he can be heavily damaged by minor explosives (grenades, small explosions) and escaped from. Birkin will just keep following you until he eventually catches you and he pretty much never goes down. If I am to die, I'd rather my death be against a monster that I can actually damage and that would kill me near instantly. Birkin would be an agonizing death due to his pipe, grabs, or claws but Nemmy would be pretty quick if he uses his tentacles which he loves to do. But I digress, Nemmy is my pick.