[jt335i]

I have 2005 Mustang GT (5-speed) in addition to my 335i. I've owned it since it was new and my GT definitely has a good burst of acceleration off the line.
Now since I'm still going through the break-in (purchased on 12/30/2009) with my 335i, I have yet to open it up and get a good feel of it, but from what I have felt in drving it, so far, it it is pretty quick off the line.
Stock for stock, I would say they're pretty close, but the Stang may have the edge. However, to get a good measure, I would have to get a friend to drive one car, pair them up, and see how they stack up off the line.
I'll back to you all on that.

Now, if you add some mods like a Dinan stage 1 or stage 2, and take on a stock 2005+ Mustang GT then, of course, the 335i will have the edge.
However, if both cars were modded, the Mustang could have the edge because there are tons different kinds of mods (Turbo, Supercharger, tunes, exhaust mods, aluminum driver shaft, intakes, etc, etc, etc.) that one can to do it and boost the HP to over 400HP. It depends one how much $$$ they want to spend on them and they're not too expensive.