depends on driver i would assume?
Both are good cars. I have owned the IS-F and have driven the M3 at a BMW event. I would have to say that I would favor the M3 over the IS-F even though the IS-F would definitely keep the pace.
In a straight line, the IS-F. otherwise, i'd put my money on the m3. it really depends on the driver, as mentioned, and which M3 you are referring to, and what kinda of mods each has. an E92 M3 might actually keep up in a straight line, im not sure.