When buying a heat pad for your dog there are 3 things you should consider...
comfort, reliability, and safety.
Since the only input you can get from them is after you purchase one, you want to think ahead in order to get the overall best one you can.
Dogs have very natural needs, so you want to choose a fabric with that in mind, something close to what they're used to as related to they're skin and fur type.
The closer it is in the look and feel of what's common to them, the more comfortable they'll be with it, and the more likely they'll use it.
When I say "look" I mean the shape actually, because dogs don't see colors, they see black, white, and shades of gray.
Having a heat pad that is reliable is even more important in certain cases.
Your dog might have to have an emergency operation and you'll want to keep them warm and comfortable during that time.
You might take a short vacation and have to leave them with a friend or relative.
It will help to know that, while you're away, they have a warm familiar reminder of the love you have for them.
Last but not least there is the issue of safety.
You want to have a safety feature like automatic shutoff.
Let's say they happen to "go", so to speak, where the shouldn't, (on the pad or near it ) you don't want the risk of shock or electricution.
Also a damaged or otherwise defective heat pad can overheat and cause serious burns to your dog.
Taking good care of your pet is what makes you and them happy, and having a happy pet brings joy to your life.