What's straight line performance? I know about this thing called handling, and I know that both the STI and the M3 were built with this in mind....
Really, the STI is a teriffic bargain performance vehicle. It does speed and handling very well (and roominess too, as a hatchback!). But in the words of Jeremy Clarkson, "the M3 does everything well". I'm of that opinion.
However, if you're just looking to go mod crazy, get the STI. The M tax is just straight up depressing.