Confirming what @ThatAPIGuy said - there's no difference at all. In the first few versions of PowerApps the property selector operator was the '!' character, but many people felt that the '.' would be better, which is why the change was made. Any document that once was created using the '!' operator, once it's opened in a recent version of PowerApps, will have it replaced by '.'.
Moving forward, please only use the '.' operator. It's possible that the support for '!' will be removed in the future.