Safest?

Both are body safe.

Both are body safe, but Silicone cannot be used on silicone toys. So it depends on your usage

Both water and silicone based lubes are safe to use. The important thing to watch out for when looking for a lube is the ingredients list. Glycerin is a very common ingredient in lubes (mostly water-based lubes) which some women have issues with (myself included). The glycerin is a sugar which feeds the bacteria in your lady parts, unbalances the pH, and can cause yeast infections. Parabens are also an ingredient which some women have reservations about when it come to product safety. There are some links between parabens and some types of cancer, but this isn't confirmed. They do mimic estrogen in the body, which is not too safe considering where you will apply it. Another issue is some women have a sensitivity to parabens, and get a horrible burning sensation when using a product containing parabens or glycerin.

If you want to stay on the safe side, there are plenty of hypoallergenic lubricants on the market, and others clearly stating they are glycerin and paraben free. Good luck!