The usual difficulty here, in comparing position players with starting pitchers. The methodology used was to pick the five nominees was simple: average out bWAR and fWAR, and pick those with the five highest numbers. It's entirely objective, but I think it covers everyone where you could make a strong case for their candidature. Here's the list, followed by a paragraph on each of the candidates, in alphabetical order.
As noted elsewhere, Cahill will be the grizzled veteran of the rotation in 2013, with more career starts than anyone else in the rotation - even if, at times, he doesn't look old enough to possess any grizz (he's 15 months younger than Miley!). He'll still only be aged 24 on Opening Day this season; the only current pitcher his age with a higher number of starts is Clayton Kershaw. He joins Daniel Hudson from last year, as the only D-backs to throw 200 innings in a season before their 25th birthday. Probably deserved a better record than 13-12, but the team scored only two runs per game in his losses, and never more than four.
Goldzilla's 123 OPS+ was the second-best by a qualifying first-baseman in franchise history (trailing only Chad Tracy's 132 in 2003 - and Chad started 51 times in the outfield). It was also the highest in the National League at the position, by a player his age, since Joey Votto's 125 in 2008. Goldschmidt also stole 18 bases, the last 1B in the NL to swipe more was Derrek Lee for the 2003 Marlins. But it wasn't all about the bat, as Goldschmidt's defense proved more than acceptable, especially given the concerns about his aspect going in to the season. But I think we know who'll be getting Timmeh's vote...
Remember when we doubted that Hill could repeat his impressive 2011 performance? 33-game small sample-size, we said? Consider those doubts erased as Hill actually increased his OPS fractionally from those numbers, putting up a line of .302/.360/.522. The resulting 131 OPS+ tied Jay Bell's 1999 as the best by a D-backs second-baseman, and he should really have made his first All-Star appearance as a National Leaguer. He'll have to settle with getting some MVP love, winning the Silver Slugger - and, of course, becoming the first major-leaguer in over eighty years to hit for the cycle twice in a season.
We hardly need to go over Miley's credentials again, having already done so as he marched his way to emphatic victories in two previous categories, but just to summarize. A franchise-best second in NL Rookie of the Year balloting, losing out to a much-more hyped player by a mere seven points, the fourth-smallest margin in the history of the award. His 4.8 fWAR is the highest number by a rookie pitcher since Hideo Nomo in 1995. His 16 wins tied Jason Jennings for most by a rookie in the past 25 years. Was the team's sole representative at the All-Star Game. Not bad, for a player that only made the roster due to Takashi Saito's dodgy calf.
Montero's 136 starts were the most by any catcher in the major leagues this year. He received MVP recognition for the second successive year, and his 88 runs batted in was second only to the eventual winner, Buster Posey. He now owns the three best offensive seasons by a catcher in Diamondbacks history (min. 200 PAs), and also set a career mark by throwing out 42% of stolen-base attempts against him this year. Perhaps most impressively, Miggy could have been forgiven for turning down the intensity after signing a five-year, $60 million contract extension in late May. But instead, he hit .298/.406/.476, for an .882 OPS the rest of the way.