I recommend the metal plate dumbbell and barbell sets over the cement filled plastic sets. First, the metal plates are smaller and super durable. I have found the cement sets to split along the seam over time, especially if they get dropped, and sometimes they feel like there can be airpockets in the weight. I have had some of the cement dust come out of the splits. They are bulky, so you can't get as much weight on the dumbell. Second, the plastic/cement sets usually come with the butterfly pincher type of collars that keep the weights on, whereas the metal dumbells come with the threaded ends and the spinning threaded collars. There is no chance of the threaded collar coming off during the exercise. The pincher type can get loose over time with use, and sometimes they actually get stiff and hard to remove. Go with the metal plates and threaded collars.