The Momentus XT is awesome. For $125 (on Amazon) you get an amazing boost of speed AND huge storage space. SSDs are faster for all your data, but not much faster for the significant stuff. Our MBPs boot in under 15 seconds, the programs we use often load near instantaneously, AND we have 750GB of disc space without having spent $750 or giving up the Superdrives (which we actually use).
The DYI Fusion drive is the way to go if you don't need the optical, but shy of that I'd take a hybrid over a small SSD & external. It's nice to have all of your stuff be portable and speed as well.