go with an 01 you wont be dissappointed.

A IRS to SRA is easy as cake and make a huge difference. The only downfall of the IRS is the reliability of the Shafts under power and weight. Other than that you can build it up to withstand and lauch as well as the SRA. You wont be dissappointed. 4.30s or 4.56s on the 01 make a whole world of difference.

A Mach 1 is going to be a little more mod responsive and quicker (stock vs. stock) due to the cams and later changed C head redesign, to an 01 Cobra. It comes with a Solid rear axle and a little better shocks/struts and ride height due to the springs.
Pricing goes any where from 12K to 20K for these cars