As with a lot of questions I get on Twitter, I answered with my gut and said Humber. Looking back, I'm not so that was the right answer.
My instincts tell me that Humber is a regression candidate, and his last two games seem to strongly support this theory. His ERA jumped from 2.69 to 3.27, and at a glance it appears that it's time to bail on Humber.
But while Humber still might have another bad start or two in him before he hits his floor, his 3.91 xFIP isn't horrible. Among A.L. qualifiers, it puts him 31st out of 50 pitchers. He might be due to slip a little more, but then again a 0.64 ERA/xFIP differential isn't an outlier. Humber's differential is 14th in the A.L. Humber could move closer to his "true" performance...but then again he might not.
Hellickson has a 3.17 ERA and a 4.36 xFIP. His differential puts him fourth among A.L. pitchers. He's much more likely to regress than Humber based on xFIP, and his xFIP is 46th in the American League.
These facts don't necessarily change my mind; there are other factors to consider. The Rays have the third best defense by UZR; the White Sox have the fourth worst. I also like Hellickson's chances of grabbing wins slightly more than I like Humber's.
However, the main reason that I might stick with Hellickson if I already had him in hand is because his pedigree is greater than Humber's ever was.
Some of the Phil Humber narrative floating around this year is that "he was a top prospect once, it just took him a while to figure it all out." There is no doubt that Humber was a very good prospect at one time, but he was never as highly regarded as Hellickson was. Humber peaked at #50 on the Baseball America rankings (way back in 2005). Hellickson ranked 18th in 2010 and 6th heading into this year. Humber is also 28 years old, while Hellickson is 24.
It's probable that what you see is what you get from Humber. On the other hand, Hellickson has room for growth and could very well develop into an ace. It doesn't look like it's going to happen this year, but there's a better chance that it happens for him at some point than it does for Humber.
The answer to Jonathan's question, then, depends a lot on your league, your format, and your needs. In a non-carryover league, Humber might very well be the better play for the rest of the year. In any kind of carryover league, though, I'd almost definitely hang on to Hellickson. Roto aces don't grow on trees, and Roto aces with cheap contracts are a surefire ticket to victory.