I don't know how a AAA assignment feels to a career major leaguer in his early 30s, but judging from these comments shortly after he signed on with the club, Scotty Podsednik isn't too keen on the prospect:
"It would be tough to answer," Podsednik said. "I've kind of set my goals to get this thing ready and to get back on the field and show these guys what I can do. I haven't really thought about it, to be honest with you."
While he stops just short of saying he'll use the opt-out, it doesn't take a whole lot of projecting to read into that non-answer. Injured foot and all, the guy needs to make the club this year or it could, as they say, be lights-out. Shawn Green was 34 when he retired, and a far superior player.
Pods and Corey "do not bat leadoff" Patterson are both above average at one skill: stealing bases. They both do it at about a 75% clip, which isn't quite Moneyball's preferred 4/5, but is still quite good. Podsednik has more career steals but a slightly worse percentage, and is 3 years older.
I've always been partial to keeping a Herb Washington around on the roster for just this purpose. DeWayne Wise served the role well last year; as long as Wise was on the roster we were able to avoid the awkward "John McDonald as pinch runner" in a critical situation. Whenever you can put a legitimate basestealing threat on first base in the late innings, your run expectancy must go up. The difference between a speedy runner on second and an average runner on first with the same number of outs is huge (provided the SB% is high enough, of course). But Herb is an extreme example; in most situations, a team is looking for a 25th man who can provide something else as well - defense, singles, power, whatever the case may be.
FanGraphs has Patterson's defense at significantly above average, while Podsednik is slightly below, according to UZR. I can't pretend to understand the mechanics of UZR, but the result accords with my preconception: there's a reason why Patterson has played almost all of his 8100 defensive innings in CF, while Pods has slightly more innings in left than in center.
But of course, defense and speed, while appreciable traits in a last-man-on-the-roster type, are not the primary skills for non-pitchers. And this is where things get interesting. Corey Patterson, of course, is absolutely atrocious at getting on base. It's the trait that has hamstrung his major league career, transformed him from former highly touted first-rounder to replacement-level player. In 4098 career plate appearances over ten years, he has 189 career walks; Jose Bautista, were he to repeat last season's performance exactly, would have walked 200 times in just 1366 PAs. Podsednik hits for a better average and takes more walks, and as a result his OBP is a manageable (and arguably, leadoff-able) .340.
On the other hand, Patterson was once seen as a potential 30-30 guy, while Podsednik is no one's idea of a masher. Remember this game? Two home runs in the 2005 playoffs, but none in the regular season, and that's pretty much par for the course. While Pods is a prime candidate to lead the league in triples, his power will never be a calling card.
So: assuming an outfield of Rivera, Davis, Snider, do you go with Pods or Patterson - or both? Carrying a fifth outfielder is not an impossibility, but with Jose Bautista, Adam Lind, and John McDonald already on the roster, someone like Mike McCoy who can play the infield would seem a more logical pick than a pure outfielder. Based on skill sets, Patterson is the more rounded player, and more useful at the skills which a pure bench guy should be relied on to contribute - he steals bases at a better clip, is a superior option as second-string CF, and a reasonable pinch-hitting option as a last resort. Pods makes more sense as a starter, platoon guy or injury replacement.
Ideally you could hold onto both, and wait for the inevitable aches and pains to emerge as the season progresses - so maybe you do leave McCoy in AAA and roll with Bautista (or Encarnacion? ugh) as a third-string middle infielder for the first month or two. Otherwise - and you have no idea how much it pains me to say this - maybe Corey Patterson is the man for the job.