To be correct:
D2C is the code name for the platform that the 2005,2006, 2007, etc. Mustang is made on.
S197 is the program name for the 2005 Mustang.
SN95 was the program name for the 1999 Mustang.
I'm not 100% sure, but I belive that Fox-4 was the Name of the platform for the 1994-2004 Mustang.
2004 Mach 1 Pros
Unique Model
Can get HUGE discounts on them
DOHC 4 cam engine puts out a bit more HP than the 2005 GT engine.
2004 Mach 1 Cons
Same old weak platform that has been used since 1994 and is based off a 1979 Fairmont.
Lots of road noise and other noises
Less than stellar ergonomics
The suspension design & geometry on ALL Mustangs from 1979 - 2004 is pretty poor, front roll center is too low, rear roll center is too high, 4 link rear end binds.
Requires 91 octane gas.
2005 Mustang GT Pros
Brand new platform, the first platform ever designed specifically for a Mustang
Quieter
Smoother ride
Better handling
Much better ergonomics
Much better suspension design
2005 Mustang GT Cons
Price - You'll pay more for a GT right now than a leftover 2004 Mach 1
For now the GT is the top model
At the risk of starting still another war here on StangNet, it appears that 0-60 and 1/4 mile times are about the same for either a Bone Stock 04 Mach 1 and a Bone Stock 05 GT.
As far as styling goes, I'll leave that to you. I like the 99-04, but love the 05.