These two products are actually quite different, and so they have their own pros and cons. Ultimately, many people prefer electric treadmills because they tend to be more substantial and have better features, such as incline adjustments, and workout programs.
Manual treadmills can be good too, but tend to be more popular with seasoned runners, because they provide more of a workout, especially when you are just starting out. This is because you need to put some energy into the machine to get it going at speed, as it moves based solely on your own motion.