I would suggest trying both the Pappy 15 and Pappy 20 before saying/assuming one is better than the other. They are 2 completly different animals.
It's not like comparing Maker's Mark and Weller Special Reserve, which are basically the same recipe and roughly the same age, just distilled by 2 differnt distilleries and aged in differnt warehouses. I would call that an apples to apples comparison.
The 2 Pappy's listed above differ dramatically in age, proof, and the combination of distillates in the bottle. The Pappy 15 is a big complex SOB that has a closer resemblence to 12 year wheaters, but just boldler in every aspect such as nose, taste, and thickness on the palate. The Pappy 20 is completly refined, tamed down a notch and unfortunately currently nothing on the market to compare it to. If I had them side by side, Pappy 20 would be my pick simply because when you are drinking it, you can taste how the wood is integrated perfectly to keep it drier/less sweet along with a nose that is unmistakably an OLD wheater. Best Sipper I have ever had.
As I have mentioned before, Buffalo Trace has done a great job (so far) in keeping to the S-W profile for the current Pappy 15 which is all Buffalo Trace. Intersted to compare my S-W Pappy 20 to the All Buffalo Trace Pappy 20 when that day comes around.