A building you own will always be best. Whether thats a stand alone or in a strip mall. For arguments sake, lets say you own both, a stand alone and a strip mall. Which is better? That depends on the other businesses in the strip mall and access to good parking. Try and stay away from liquor stores and bars, they can be trouble neighbors, at least thats my experience with them.
Have you ever noticed that when a Walmart is built that other businesses will build nearby or in the out lots of that Walmart. These other businesses share the same customer as Walmart, same demographic. So if Walmart builds, they build. So if your strip mall has ample parking, a good demographic and other businesses that share that same demographic, it can do really good. They can all feed off of each other and help each other.
On the other hand a Laundromat is a destination business so a stand alone can also do great.
The correct answer is, as long as you run the best laundromat in your area, you will win. Regardless if its stand alone or in a strip mall.