The stock 94-98 Shorties aren't quite as good as true aftermarket but the difference is too close to warrant spending the money unless you go SC or use a STS turbo system (Turbo at rear of car, don't have to get special manifolds.)
The aftermarket guys usually don't say much about their 99-04 collector size because they have to leave them stock (2") so that people can use them with their stock Y-pipes. When you do decide to get a set of aftermarket headers be sure to get 94-98 shorties because they will have the 2 1/4" exhaust, you will have to change your Y-Pipe to use this larger size.
In fact anyone that has a 99-04 using 94-98 Headers must either have a shop enlarge the inlet of the Y-pipe to 2 1/4" or purchases a Y-Pipe from a 94-98. It would be cheaper to have the shop do it. I was charged $5for eachexpansionon acustom exhaust setupfor my truck, CHEAP!
Now I am not sure if the exhaust diameter itself shrunk in 99-04 or if it was just the inlet that changed. I can do a little more research into that.
Ultimately its still far cheaper than aftermarket headers
You can also have the 94-98 Ceramic coated like I had mine done. Ceramic coating the inside of them is very important too to help the exhaust flow through and to aid it scavenging. This process can become a little expensive.
Now for the main benefit of the 94-98 headers is that they are 3-1 design which promotes horsepower and torque, the primaries are almost 1/4" larger, the Collector is 1/4" larger and these don't crack