Whats the difference between tele-zoom macro and prime macro?

As well as the quality issue, there's magnification. The Sigma 28-300mm f/3.5-6.3 DG IF Macro you mention gives a maximum 1:3 magnification. This is the same as the Canon 18-55mm f3.5-f5.6 IS kit lens (which isn't actively marketed as a "macro" lens).

The Canon 100mm macro (and almost all other prime macro lenses from Canon, Sigma and Tamron) give a maximum 1:1 magnification - a 3x improvement. They're also a couple of stops faster at that magnification, which means a clearer, easier to focus viewfinder image.

Close focusing zooms can give a useful introduction to close-up photography with DSLRs, but once you get serious about macro, a prime macro lens will get you closer with much better quality.

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Check out my galleries (lots of macros) at :
http://www.pbase.com/cjed