For very many reasons, I much prefer working with Acrylic yarns for most of my projects. I find the acrylics are much softer and easier to work with. Most cotton yarns are much finer, thinner, and rougher to the touch. They do get softer with repeated washings, however. Cotton also shrinks much more than acrylic. My fingers are not long and delicate and I find working with the thinner cotton yarns to be much more tedious and frustrating. Also, natural fibers tend to be much more expensive than acrylics. Acrylics also tend to keep their shapes and colors better than natural fibers, especially with repeated washing. 

For most projects, the choice of yarn is mostly a personal preference. Hot pads, however, have a bigger consideration – safety. If you do a deep dive on the internet, there are many different and confusing caveats!  The general consensus is that cotton or wool should always be used for hot pads. This is especially true of anything (cast iron, pizza or bread pans, etc) that might be heated to very high temperatures. 

Many other crocheters have said they have been using acrylics for many years and never had an issue with normal use – ie pans out of a moderate oven or off the stove. 

The other issue is not so much about safety as it is about use. For dishcloths or coasters, cotton is more absorbant than acrylics. 

I do crochet all of my hot pads with either thick chunky yarn or double strands so they are extra thick. That offers more durability and safety.