Welcome to the site!!
In a nutshell..
99-04 GTs were sweet, I had an 02 GT that I miss dearly.. the 99s-02s used a t45 transmission in manual trim, later models had a tremec tr3650 trans
The bullitt models were alright, a slight bump in HP, great suspension work, no fog lights or spoiler and a unique color (ever see the movie bullitt?) Unique and decent handling but...
99/01 cobras had several visual and interior upgrades, suspension upgrades, as well as a 4V DOHC version of the 4.6 under the hood
The 03/04 mach 1 had a sweet suspension setup, like the bullitt, tokico shocks, etc had a nice version of the 4V DOHC 4.6 (like the early cobras) solid rear axle, a sweet visual theme, flat black highlights and a shaker hood. I would get a mach 1 myself, and I plan to once I get out of college and start working full-time, also had some model-unique colors like azure blue
The 03/04 cobras were the beasts of the bunch, supercharged 4.6 DOHC motors, plenty of suspension work, independent rear suspension to boot (not the best for the drag strip, but made it handle great) also used a t56 (I believe) 6 speed trans
2012 IS F Blue IS F Sport
2006 Mustang V6 wannabe