You should get the A2. The BDD451 is for standard, but it's highly aggressive. The BDD451A is for automatics, but can be used on a standard because many consider the BDD451 too aggressive. (I have the BDD451A) The BDD451A2 is for those who have an SRT it has a better throttle response curve designed for the SRT, but some say it's less aggressive then the even the 'A'. There's a lot of information over at MBWorld.org.