From a performance standpoint, both Rick Porcello and Drew Smyly have a strong case to be made for the job of fifth starter on the team. Both posted FIP's and xFIP's (fielding-independent pitching, on the same scale as ERA) just under four. Smyly got there by relying more on strikeouts (8.5 per nine innings) while Porcello relied heavily on ground balls (53%, 21 double plays).
Smyly's ERA was nearly a half point lower than Porcello's in his rookie season, but Porcello posted a similar ERA in his debut season as well, and has decreased his FIP each year since.
There are other factors that could be taken into account, but realistically, should be ignored. Porcello has more experience with four full seasons under his belt, including a handful of playoff starts, along with a start as a rookie in game 163 back in 2009. But there's no reason Smyly can't build that experience over time, and while Porcello has shown improvement, it's likely that improvement was due to him improving his skill-set in a prime development age (20-23) as opposed to experience benefitting him.
Multiple individuals have argued that Smyly would be a better fit because he's a left-hander in a rotation full of right-handers. While some managers may take such a thing into account, the most important piece in examining this theoretical competition is, who does a better job of getting outs? Perhaps left-hander vs. right-hander could be a tie-breaker, should all other variables net out even. However, choosing an inferior pitcher for a role as a starting pitcher because he throws with the other hand is rather foolish.
From a contractual and financial standpoint, this clearly favors Smyly. As a second year player, Smyly is still two years away from arbitration and won't be eligible for free agency until after the 2018 season. That's many future years of affordable production. Just looking at this year, Smyly's contract will be renewed at a minimal salary in the neighborhood of $500,000. Comparatively, Porcello was a super two last year (eligible for arbitration), will be due another raise this year in the neighborhood of $4 million, and will be a free agent after 2015. So, Porcello will likely cost in the neighborhood of eight times more than Smyly, in addition to being able to garner a big free agent contract much sooner as well. Of course, $4 million vs. $500,000 isn't all that much when considering the Tigers payroll for 2013 will be in the neighborhood of $150 million.
Availability to contribute is also an important factor that needs to be taken into account. In four years of big league service time, Porcello has never once gone on the disabled list, nor has he ever experienced any significant injury. He was sidelined for the final month of his lone minor league season, but that was due to non-baseball-related reasons. When it comes to pitching, Porcello has been a model of health.
Smyly on the other hand hasn't had the same durability. He missed his entire freshman season at Arkansas with a stress fracture, and he was sidelined in 2011 with elbow soreness, making only 21 starts over the year. He also spent two weeks on the disabled list in 2012 due to a blister on a finger on his throwing hand. These sorts of durability concerns cannot be overlooked, as missing even a handful of starts, especially without a reliable alternative, can significantly drop the value received by the Tigers for the fifth starter's spot.
Looking at "fit" with the Tigers, it's no secret that Porcello hasn't been the ideal arm for a team with an infield defense like the Tigers. Miguel Cabrera and Prince Fielder are both below average defensively, while Jhonny Peralta has limited range at shortstop. It's a contributing reason why Porcello saw his average on balls in play jump to .344 in 2012, the highest in MLB. And just to counter some arguments that Porcello just served up easy to hit balls, Max Scherzer's BABIP at .333 was second in MLB, and the Tigers overall were fourth in baseball at .307. The Tigers don't convert ground balls into outs as much as other teams do, and that's a problem for a pitcher that only averages 5.5 strikeouts per nine innings, and has more than half of his balls in play on the ground.
From a ceiling standpoint, as counter-intuitive as it may sound, there's a good argument that the ceiling with Porcello remains higher. His troubles remain two-fold. One, he has never seen the development of a go-to out pitch, constantly waffling between a curveball and a slider over the years (and much of this fault rests at the feet of the Tigers, it's worth noting). Second, he appears to tire quickly, with an OPS against ballooning to over 1.000 after the fifth inning as well as on pitches 76 and up.
On the former trait there remains hope; manager Jim Leyland told Lynn Henning earlier in December that Porcello's slider and curve are both strong pitches in his bullpen sessions, but they lose their effectiveness when on the mound in live games. The fix is easier said than done, but the hope remains that a strong slider paired with his sinker would make him a very good, middle of the rotation pitcher. That would also put him closer to the pitcher many projected when he was one of the top prospects in baseball. If that doesn't come about, he'll remain a durable back of the rotation starter, dependent on his defense to get him outs.
Smyly on the other hand has less than a full season of experience, but might be much closer to his ceiling than some might be willing to admit. His four pitch mix (fastball, slider, curve, change-up) is effective based on his command of those pitches, along with mixing them (including two-, four- and cut fastballs) to keep hitters off-balance. His velocity sits around 91, and none of his breaking pitches will likely ever be true plus pitches.
The story comes together with two pitchers with their strengths and weaknesses, and no clear cut winner. Smyly is the cheaper option that brings a better ability to get outs on his own, but has durability concerns and likely won't elevate past the level of a number four starter. Porcello on the other hand won't come cheap, and will hit free agency much sooner. But he's a good bet to turn in 180 or more innings across 30 starts for a season, and he still has the potential to take another step forward in his development.
With no obvious holes in the lineup or on the club, there shouldn't be a strong urgency to move either pitcher at this point in time. But if they do elect to part with one, it's hard to say which will be the better move for the club, and could well come down to which pitcher nets the best package in return, as Smyly is likely the more cost effective option, but might not be the best on the field.