The Tamron has several advantages.
First, with the current $100 rebate it's cheaper. The
Tamron 60mm f2 macro is $399 after rebate from Amazon. The Canon sells for $449.
Second it's a full stop faster with an f2.0 maximum aperture vs. the f2.8 of the
Canon EF-S 60/2.8 macro . For macro work this might give you a slightly brighter viewfinder and possibly better AF accuracy, though since you'll likely be shooting stopped down to get DOF there's no real optical advantage. However if you use the lens for portrait work too (and 60mm f2 makes for a good portrait lens), then you may well shoot it wide open for the smaller DOF and greater background blur.
Third, though it's a crop sensor lens it doesn't use the Canon EF-S mount which means you can mount it on a full frame camera and you can use a TC like the
with it to give you an 84/2.8 lens. You can't do either with the Canon EF-S 60mm macro due to the EF-S design mount.
Fourth, the Tamron comes with a 6 year warranty (vs. 1 year for the Canon) and is supplied with a lens hood (with the Canon you have to buy the hood seperately).
I've never tested them side by side and the Canon is a very nice lens, but on balance if I was looking for a 60mm macro for a crop sensor camera, I'd probably go with the Tamron.