Having a tarp that is both mildew resistant and water resistant is great. Anything having to do with outdoor applications needs to have these features. This is because the tarp is the item that is designed to protect your other belongings. A tarp is useless unless it posses the qualities of being weather resistant. A good polyester tarp is what you want to get.
10x10 is a fantastic size. It functions well and is capable of covering most of the objects that you will want to protect from the elements. Many times people use these tarps to cover sensitive machinery that needs to be left outside for extended periods. This is a perfect size and can easily encompass most anything you need.
The vinyl tarps are also fantastic because it can easily be accommodated for many applications. When not in use it can be folded up and stored in a separate location until it is needed again. This is the benefit of using a tarp as opposed to a more rigid covering.
Of course, these vinyl tarps are also great because they can be washed and cleaned. If they are on the ground and are being used to protect a workspace then they will become dirty. But the design allows them to be easily cleaned. This is good because if they have been sitting outside covering a piece of furniture or a machine, then they will likely have a top surface which is dirty and sometimes filthy. You would not want something that would need to be thrown away. Rather, you want something that can be cleaned and then stored for later use.