It all depends on what kind of music you are going to produce. If you are producing up to 48 track stuff with an average number of plug-ins.. then an MBP is fine.. (maxed out with RAM of course) Instead of a huge hard drive, I would recommend a hard drive that spins at 7200 rpm.
You can always connect an external firewire drive (7200 rpm) that has more gigabyte space to record your sessions, and to store them (also keep a backup drive)
I prefer the mac pro for my production needs because most of my stuff goes beyond 64 tracks. I also tend to use a lot of plug-ins. For all that, I record on 10,000 and 15,000 rpm SCSI drives. Additionally, for the lowest latency and a general solid audio performance.. I prefer a PCI audio interface as compared to the plethora of firewire/usb interfaces out there. Firewire/usb interfaces are affordable, and work great.. but if you want ultimate performance and no freezes or slowdowns.. PCI can't be beat.
There *is* a solution to work with an MBP and SCSI drive and a PCI interface. Check out
http://www.magma.com. They sell pci enclosures.. and they connect to the mbp via an expresscard (although the current one only has 1 pci slot.. good for a scsi controller card)
On the MacPro using SCSI drives and PCI interfaces.. I've gone upto 256 tracks with plugins, virtual instruments, etc. and the system performed flawlessly.
If you're just starting out though, I would recommend an MBP.. as its a laptop.. you will take it everywhere with you. Its a lot more convenient to sit on your bed and work on a session on your laptop.. than confine yourself to a room where the MP is.