I would think about getting the 3.33 6 core machine. If you are primarily working with audio tracks and effects plugins, you will likely not run the 6 core into the ditch unless you have crazy plug counts.
If you are primarily working with VI's (soundtrack composer for instance), you could achieve the same or better performance for less money using VSPro and slaves (either Mac or Windows) potentially.
I have quite a bit of experience with this (I run Logic, VEPro, Kontakt, Omnisphere and UAD on a 3.33 6-core with a Mac Mini server slave), so if you could be more specific about exactly what you are doing and what tools you use, we could help you better.