The Scoche fd-15 is the only completely correct harness for bypassing the amp. If all you're looking to do is bypass the amp, that's all you need.
The other two numbers I imagine you see are the Metra kits to install an aftermarket HU. There's one that has the factory stereo harneses and an incorrect amp bypass cable (#5511 I think) and another that is not for the premium system that only has the factory stereo harnesses and has no bypass cable at all (I don't know the number on that one).
If you're looking to install a new HU, the very simplest way to go is to buy the non-premium kit from Metra and the Scoche FD-15 bypass harness. Follow the directions on each and it's just plug (and solder) and play, using the Metra harnesses for the HU and the Scoche for the amp bypass.
You can get by with just the 5511 from Metra, but it requires a little extra wire and soldering. If you cut the correct plug off of the amp bypass (test fit them both and it's easy to spot the right one) and solder it to the factory speaker harness with some extra wire inbetween, you can just plug this new cable into the new HU and the amp cable that matches (it goes to the speakers). Basically you're just bypassing the cable from the HU to the old amp AND the old amp this way, instead of re-using the old wire from the HU to the amp and only bypassing the amp.
It all sounds a lot more complicated than it is. If you spend a few minutes looking at the stock setup and realize where the wires come from and go to, you'll see what your options are.
Feel free to ask more questions or for clarification. I know it's confusing to read.
