I am told chlorinated works better in lower temperatures. Pick your poison is right - my understanding is that both are harmful to your body, but due to different chemicals.
Actually the doctor ordered a little bit more info:
Standard brake cleaner was always made with TCE's (tri-cholorylethylene, I believe), which is a man-made solvent that has numerous bad effects on your body. One of its prinicipal problems is that it collects in your reproductive system and can permanantly alter the DNA in your reproductive cells. Additionally, environmental exposure to this solvent, simply from breathing the air inside a car shop while at work, has recently been found to be more serious than ever thought.
Newer brake cleaners don't have chlorinated solvents, but I'm sure some of the chemicals in them are not ones you'd want to be around daily. At least they don't collect inside of you forever and constantly cause damage!