The MP-E 65 is a tricky lens to use. While it does excel at extreme macro it is useless for anything else.
The Canon 100 mm macro goes only to 1:1 (more with the EF25 extension tube). But it has autofocus and is perfectly usable for regular photography.
Extension tubes rob you of some light, but not of image quality. The 25mm extension tube coupled with your f1.8 50mm plastic fantastic (a lens every Canon shooter should have in his bag) produces excellent results.
Closeup lenses such as Raynox tend to rob you of image quality, but with good achromatic design this is manageable. While I have not used the DCR250 personally it has solid reputation and respectable following on flickr.
Stay away from single element closeup lens. +1 diopter is OK, but it is not true macro - it works best when you want a tight portrait with a long lens. Stronger single element lens introduce chromatic aberration that is painful to watch.