First off understand that the 50mm f2.5 macro is not a true macro lens - it has macro in its name but it requires the addition of a specific lifesize converter being added to it before it is able to achieve the same level of magnification that the other true macro lenses can produce. The costs of both parts together generally results in a higher price than some other longer focal length options such as the 100mm macro.
For a cheaper macro lens you might want to look at the sigma 70mm macro, sigma 105mm macro, Tokina 60mm macro and the tamron 90mm macro as well as the canon 100mm macro.
Essentially they are all as sharp as each other - minor variations that might show up in studio tests won't really affect real world shooting and the differences shown can be down to sample variation rather than inherently different optical performances.
In addition with regard to magnification each true macro lens will give you the exact same framing when focused at their closest focusing point - from 60mm to 200mm the shot at the closest point will be the same - so you don't gain any magnification with differing focal lengths (though longer focal length lenses will render more of the background as blurr).
The main differences are thus price -features and focal length - the focal length being key to understand because longer focal lengths generally give you more distance between the camera and the subject. For insect work this is often a bonus as it means you're further away when taking the shot and thus less likely to spook the insect.