.sig files are binary files while .asc files use the ASCII encoding and are human-readable. The information they are containing is the same.
When you use Kleopatra to sign a file it creates a .sig file by default. When you open it with a text editor you can see some “weird” signs.
In Settings → Configure Kleopatra… → Crypto Operations you can enable “Create signed or encrypted files as text files.” Then Kleopatra will create .asc files. When you open those with a text editor you will see that it contains normal characters which you are able to read.