.htm came about when Windows could not handle more than a 3 letter file name extension. *nix and the internet uses .html and so should you.
Other then the obvious, the letter "L," there's not much of a difference between the two extensions. Most, if not all, web browsers and servers will treat a file with an HTM extension exactly as it would a file with an HTML extension, and vice versa.
For user there is no difference using .HTML or .HTM that just because the difference naming conversion
it's the same as with *.jpg and *.jpeg. Always use .html, the three letter extension was due to Windows 8.3 filenaming convention used in DOS because it could not understand 4 letter extensions or long filenames. html is the proper extension and should always try to use this instead of htm. However, if you have an index.htm and index.html on you web server, one of them will have the advantage and most probably index.html as almost all host servers will require your default starting page to be named as "index.html" and not as "index.htm". Although, that will also vary depending on your host and their setup.