The STi driveline is exactly what it suggests - the driveline (including transmission, rear diff, driveshaft, and half shafts + hubs) from the STi
The STi transmission is pretty damn strong, but the PPGs are stronger. The advantage of the STi driveline is that you may get the DCCD and limited-slip front diff depending on the version of the STi gearbox. The disadvantage is that it is not as strong as the PPGs, and may require a lot more work... and work = $$
I want to add that if you can find a JDM 6MT STi transmission, you get the advantage of the stronger gearset, and can utilize the stock half-shafts + hubs, driveshaft, and rear diff... but in most cases you sacrifice the DCCD for an improved 12KG viscous center diff.