There are definite advantages to going with the kettle and it all hinges on the diameter.
A 15g kettle will have a larger diameter than the half barrel converted keg. That means that the boil off will be slightly better and the ability to keep the trub in the center of the pot after whirlpooling will be better which allows you to drain off more clear wort around the periphery. Of course, you have to set up your wort drain to draw around the periphery and not the center. The center is where all the trub is if you're doing it right and you don't want to draw off there. The final reason is that the larger kettle diameter improves the transfer of heat from the flame to the wort.
As for trub settling, I can see the larger diameter being helpful. On the flip side, I can see the shape of the keg being more favorable as well.
For heat transfer, I thought the chimb on the keg would help keep some heat in that is lost to the outside of a kettle. Or the chimb might just heat up and radiate more heat away. As for size, if the size of the burner isn't huge, I wouldn't expect the big kettle to really help there either. Now the kettle could have thicker bottom or a different metal sandwiched between layers of stainless to help distribute the heat. I think we really have to do a test to resolve this one. Two pots, same burner configuration, which boils faster?