That the Cardinals have sent Chris Perez to AAA in favor of keeping Brad Thompson in the bullpen, demonstrates that in the roster formula for LaRussa and Mozeliak major league experience and the role a player is in (long term relief for Thompson) matter at least as much as superior talent (Perez vs. Thompson).

It seems a good bet that same formula will be applied in assigning one of these players to AAA:

• Thurston
• Ryan
• Barden
• Rasmus

The best move, IMO, would be to send Rasmus to AAA for a couple more months of seasoning, for these reasons:

• Just as the Cards have a surplus of RH relief pitchers, making it possible to send Perez to AAA to work on his conditioning and secondar pitches, the Cards already have a surplus of lefthanded outfielders, including Ankiel (full time) and Duncan (full time vs. LHP at least), and Skip Shumaker is said to be in line for a shift to the OF late in games when he is replaced by a reserve infielder with more experience at 2B (Ryan or Thurston or, best of all defensively, Barden)
• The market value for Ankiel and Duncan is not as high as it will be in a couple of months, if they show they are in full health (Duncan recovering from neck surgery, Ankiel from abdominal surgery).  Giving both maximum playing would be the best way to set up the most favorable trade of one of them before the July trading deadline.  Rasmus could then step into the role Rick or Chris vacate. 
• Rasmus, like Perez, has shown that he needs more seasoning to harness his own talent.  His hitting has been inconsistent this spring, despite flashes that show off his ample talents.  And he has even misplayed some balls in the field.  Giving Colby a tuneup at AAA and then moving him to the big leagues this summer would put him under less pressure than putting him on the opening day roster.
• The clock on the arbitration and free agent clock for Rasmus would be delayed a year if Colby came to the Cards this summer rather than in April.   That alone would mean that the Cardinals would be spending a whole year of Rasmus at low cost just for his playing part time for two months this season.  Mozeliak seems too smart at managing the team's assets to squander his player resources in that way.

So it shouldn't be a surprise if the Cardinals start the season with these players instead of Rasmus:

• Thurston - the only LH infield reserve with much power
• Mather - the only RH outfield reserve with much power and one who could platoon with Ducan vs. LHP
• Ryan - the best reserve at SS and a solid defensive reserve for Shumaker at 2B, with a couple of years of ML experience already
• Barden - another RH bat off the bench who could then sub in the field at 2B, 3B, or SS, with very good hitting this spring and a taste of ML experience, at least

The choice really seems to be between Barden and Schumaker.  A case can easily be made for keeping Rasmus and sending Barden to AAA, of course.  We might not know which way the Cards go until the end of the last spring training game: a nice little drama to make this last week of waiting for the real games to begin a bit less boring....