I haven't owned a Mustang but my understanding is that the performance will be very similar. Although, your Mustang may feel faster. The M3 has a way of delivering the power so smoothly and linearly that it doesn't feel fast (the speedo will be your only clue!).
The biggest differences will be in quality, comfort, aesthetics, etc. All of the non-performance stuff. The M3 is the hands down winner here.
If the Mustang doesn't cut it for you in the non-performance areas, I say go for it. It's not worth any other way.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R