By contrast Trevor Plouffe's name usually comes up in the context of "failed" Twins draft choices. He has moved rapidly through the system despite lack of outstanding numbers at any level. He is in his second year at AAA and he started the season by hitting under .225 in April and May and was still under .250 at the allstar break.
The result is that Valencia is hitting .296 .317 .457 at AAA while Plouffe is hitting .259 .310 .402. It seems clear that Valencia is more ready than Plouffe to contribute at the major league level.
Unfortunately, those numbers are highly misleading. Afterall, the minor leagues are about development - its how a player ends the season, not how they start it, that determines how successful a year they had. And when you compare Valencia to Plouffe post allstar break, Plouffe has been doing much better.
Valencia's second half numbers are .265 .291 .400 - nowhere near what you would like to see from a guy you are ready to hand third base to, even if it were against major league pitching. Valencia had a bad slump in there, but he has not been tearing up the league even since that slump ended. It looks like his AAA numbers are inflated by a hot start.
Plouffe, by contrast, has been much better offensively in the second half. His numbers are .286 .333 .466. since the allstar break. And in August he is .309 .370 .543. Those August results are probably a little misleading due to the small sample size that allows them to be skewed by one recent game where he went 4 for 4. But he has hit over .270 every month since May.
Plouffe, who is two years younger than Valencia, looks like he has stepped up this year. Valencia looks like he may have hit a wall at AAA. Of course, that is based on half a season. Valencia may still be able to adjust and step up into a major league role. But, right now, Plouffe has given a lot more evidence his bat is almost ready for the major leagues.
There is still the question of whether Plouffe's glove is ready to let him play shortstop which is a defensive position. Plouffe has made a lot of errors while mostly playing shortstop. But he has also had time at third base in his career. Valencia is strictly a third baseman. So its hard to know which would be the better defender at third. But if you were going to roll the dice with a young player at third next year, Plouffe looks to be ahead of Valencia at this point.