I have major holdings in both COS and PWT. COS probably has more upside in equity price if sentiment changes back to worrying about oil supply disruptions and shortages. The distribution future is hard to predict, but could reach as high as a 8-10% return on a price of US$25. There is uncertainty regarding the coker odor fix, but I am confident they will fix it within a few months. PWT is the safer bet on distributions. You are likely to get a steady level or slight increase unless oil prices crash. They are nearly 50% hedged in both oil and gas so even the current softness in NG price won't hurt them until next year if NG stays low.
So for safety of distribution, PWT, and for price upside, COS.