I prefer the ATAK, more aggressive. It's all dependent on what you want to spend, and if you plan to go long tubes. Check out my thread, I reviewed the axle back ATAK with no NPP valves:
http://www.camaro6.com/forums/showthread.php?t=476988
If you get the secondary cat delete, besides the crushed pipe portion at the end, you really don't need the midpipe IMHO. Your SS already has a crossover, and I seriously doubt another xpipe will make much of a difference in power or sound, if any. Again, if money is no object, the valves and crossover are great quality products from Borla. Good luck!