An in-depth breakdown of their strengths, weaknesses, contracts and injury histories should shed some light on this question.
James Shields (Tampa Bay Rays)
Career stats: 87-73, 3.89 ERA, 1.22 WHIP in 218 GS.
2012 stats: 15-10, 3.52 ERA, 1.17 WHIP, 227.2 IP, 58 BB, 223 SO.
Contract: team options for 2013 ($9 million or $1.5 million buyout) and 2014 ($12 million of $1 million buyout).
Shields is among the most durable starting pitchers in baseball. Since his first full MLB season in 2007, he has amassed 1364.1 innings (postseason included). That's the fourth-highest total, trailing only CC Sabathia, Justin Verlander and Roy Halladay.
He's approaching his 31st birthday, but has never spent time on the disabled list.
Shields masterfully mixes his pitches. Though the changeup is his bread and butter, he frequently throws cutters and curveballs, too.
He predictably regressed this past season after surprising as an AL Cy Young candidate in 2011. Shields tossed three complete games (down from 11) and averaged fewer than seven innings per start. His quick move to first nabbed only three unsuspecting players.
On the other hand, seldom use of his fastball led to increases in velocity and strikeout rate (via FanGraphs).
Shields would've been considered an ace in most MLB rotations. The Rays used him as their No. 2 option behind David Price.
He's a nice bargain under the terms of this deal, which is why Tampa Bay can demand a talented package of prospects in return.
At the non-waiver trade deadline, Jon Heyman of CBSSports.com reported an asking price exceeding what the Milwaukee Brewers sought for Zack Greinke. That has presumably dropped now that the buyer would control Shields for only two seasons.
Josh Johnson (Miami Marlins)
Career stats: 56-37, 3.15 ERA, 1.23 WHIP in 144 GS.
2012 stats: 8-14, 3.81 ERA, 1.28 WHIP, 191.1 IP, 65 BB, 165 SO.
Contract: $13.75 million in 2013.
The 28-year-old has slightly more MLB service time than Shields, but 74 fewer career starts. That disparity is attributable to Johnson's list of health issues. Since contending for National League Rookie of the Year in 2006, he has missed time due to Tommy John surgery, back pain and shoulder inflammation.
JJ captured the NL ERA title in 2010 and may have repeated the feat in 2011 had he avoided injury. He rarely surrenders home runs—only 59 allowed in 916.2 IP—or plunks opposing batters.
Inefficiency is his chief weakness. This desired trade target consistently spends four pitches per plate appearance and has never pitched a major league shutout.
Miami's poor fielding skewed Johnson's 2012 numbers (3.81 ERA vs. 3.40 FIP), while weak run support and relief damaged his win-loss record. Still, several peculiarities stick out.
Johnson didn't record a single complete game. His diminished fastball contributed to a less-than-stellar strikeout rate and left him susceptible to theft. Of the 30 base runners to attempt a steal against him, 90 percent slid in safely.
At least he ended on a positive note with five consecutive quality starts.
Unlike Tampa Bay, the Marlins aren't poised to contend next season. JJ attracts a crowd to the ballpark, but doesn't comply with the franchise's desire to lower payroll (via Clark Spencer, The Miami Herald).
Which pitcher is the better trade target for MLB contenders?
Miami might absorb a portion of his fully-guaranteed salary to receive high-ceiling prospects.
So...Shields or Johnson?
Shields doesn't fit with the New York Yankees or Texas Rangers considering his tendency to give up homers. Johnson, meanwhile, would be a risky acquisition for any American League club because he's not accustomed to facing designated hitters.
Ultimately, Shields is the much safer choice. Teams know he'll be available every fifth day and regularly attacking the strike zone. Let's not forget about his pennant-race experience that includes a scoreless victory in the 2008 World Series.
You can hear phones ringing in the Rays' front office already.