Whether to sell would depend on your tax situation (in an IRA, or if in a regular account how much deferred taxes you'd have to pay).
As to whether it can keep going higher, the yield is now under 4%. That's the very low end that MLP yields tend to touch. So it would likely only go higher based on distribution increases. But how much of a higher distribution can ETP squeeze out of this without making the coverage ratio too low? So what you need to look at is the DCF expectations and figure how much ETP might be able to raise SXL;s distribution, then consider that percentage as the top end of price appreciation you are likely to see. And that is assuming everything goes perfect. Any disappointments and this could fall quite hard. It's priced for perfection.
In conclusion, whether to sell is impossible to recommend without knowing your tax situation. But I wouldn't be a buyer at this price.
Thanks for your reply. What you write is pretty much my exact feelings about this stock. I really think that this stock is priced pretty much for perfection; and after looking at where we are, I deced to sell at $52.70. I think we'll probably hover between $49 and $50 after this runup.