I checked it out real quick, 304 has better corrosion resistance then 409 and is less metallic. 304 will not discolor from the atmosphere alone, 409 will. 304 is just a better quality metal, that will keep its appearance better. All the exhausts u mentioned sound great from the clips I have heard, but its still hard to tell without actually hearing it in person. I actually just bought the flowmaster kit and am impatiently waiting for it to arrive next week. Found it in an ebay store for 650, and there is a few more available. I figure for the money it should be pretty good.
IBM 2SS/RS, M6, Blk Leather...
CAI, ARH Headers, Flowmasters, Diablosport tuned, Barton short-throw
Ordered 3/19 Delivered 8/12 VIN# 21970
94' Z28 sold 12/14/09