I would get the 100mm macro. If you do make sure it is the model with USM.
If You go the extension tube route, make sure they are EF-S compatible - the Canon models that are EF-S compatible have a "II" after them. That way they can be used with all the Canon lenses you own (or will own).
If you get an external flash, it will work much better for macro if you also get an off camera shoe cord. The OCSC is really the only way the flash will be useful for true macro, without getting special flash units for macro.