This question is designed to be the successor to Why don't most animals have "heads" in the middle of their bodies?
The previous question was flawed as it fails to accurately define what constitutes a "head".
Keeping on topic, the question here is largely focused on placement of brains and brain-like structures.
Intuitively, common sense would dictate keeping the brain roughly equidistant from everything it controls, decreasing the number of potential failure points between the brain and peripherals.
The standard argument in favor of putting the brain in the head rather than in the middle is for dedicated specialized visual processing. This argument is easily debunked seeing that there's no reason why you couldn't have a brain in the middle of the body and a smaller dedicated optical co-processor in the head. (similar to how a certain premium laptop design 7 years ago had only integrated video built-in but comes with a dock that supplies a high-end external video card). The smaller head-brain would run all of the intensive processing and send a compact representation to the central brain, adding a modest amount of extra reaction time lag. Extra credit if the central brain can partially offload high-level or heavily spatial thought processes to the smaller head-brain (think of it as using a CPU and GPU working together to solve a problem more quickly than the CPU could on its own).