I'm not sure there is a difference (both hit PHP and neither will recreate the image if it exists already), but thumb.php will result in an error if the size is too large, while Special:Redirect will return the original file, so probably that's the cleanest solution:

https://commons.wikimedia.org/w/index.php?title=Special:Redirect/file/30C3_Commons_Machinery_2.jpg&width=640

(this form will work better if the file name contains a question mark)