I notice that RMagick seems to be mostly used for making thumbnails of JPEG images for web pages, so I thought it might be useful to do some benchmarking of the 4 resizing methods in RMagick. The results are at http://rmagick.rubyforge.org/resizing-methods.html. If this is something that interests you, please give it a read and let me know what you think.
P.S. If you just want the quick answer, here it is: don't use resize, use thumbnail.
P.P.S However, if you don't go to the web page, you won't get to see a picture of me at the Grand Canyon 