For macro, ff is better, because you get the detail. But since you loose the 1.6x crop factor, you have less zoom, so you may disturb bugs, and, you technically loose magnification, since your t2i gives you a (example 100mm macro) 160mm 2.8 with minimum focusing distance of x. With ff, you'll have a 100mm zoom, and the same x focusing distance.
Technically, no. A 100mm macro lens focused at the MFD will project the same sized image on both a FF and an APS-C sensor. There's no additional real magnification, the smaller sensor is just capturing a smaller portion of the image circle. Depending on the pixel densities involved, the APS-C sensor may (and likely will, for a newer APS-C camera) put more pixels on the target. But if that means more magnification, then a 5DIII has more magnification than a 6D, because it has a higher pixel density. If you compare a FF 5DII to an APS-C 20D with a 100mm macro lens at 1:1, the subject would be the same size in both images, because the pixel density is the same for the two sensors.
+1. I was going to say the same thing when I read this response, but good thing Neuro saw it first and explained it more succinctly than I could have.
Another issue for me was that I always wish for higher ISO sensitivity since the close focusing of a macro means less light available (and I don't own the macro ring light). So I think the FF advantage would help here as well.