It depends on what you’re developing the theme for.
Premium Themes – Themes for the masses.
If you’re going to sell or distribute the theme to many people use the template drop down which provides the most flexibility to the user. If they delete the page or rename the slug they can always reassign the template using that drop down.
Custom One-Off Theme – Theme for only 1 client.
That being said, in some instances it may just be easier and more user friendly if you instead limit that option so not to confuse the individual. You could save the static page ID as an option or global variable, whatever your preference is and refer to it when necessary. You could combine the best of both and create a “Theme Options” page which allows the user to choose which page they want for a specific feature, then save the page ID; similar to assigning Front-Page or Blog ( WP Settings -> Reading ).
There’s been times when I’ve used option 2 but in most cases the page template drop down suffices and is probably most preferable.