whether you need the 180 over the 100 depends on what you're shooting and whether you're using a full frame camera - i suggest you do a search as there have been a number of discussions on these lenses
i like the 100mm - reasonable size, price, and an excellent lens. i would not recommend a simple diopter with this lens, but the Canon 500D will maintain IQ. the Kenko pro 1.4x converter also works well with the lens and maintains autofocus (the Canon extenders do not work) - on a 40D this results in a 224mm equivalent (of course the lens will be a 160 equivalent on the 40D).
to maintain focus range, i prefer to use the extender, then add the 500D rather than use extension tubes