Taveras didn't stay in the minors past age 22, and power blooms much later than that.  Still, ages 21 and 22, Taveras had 4 HR and Francisco 26, in about the same number of games and at the same levels, so the difference is clear enough.  (Just to make sure we're comparing apples to apples, I'm going to adjust everything in the minors to per-150-games.)

But Francisco's clip of 18 HR/150 games is not exactly impressive, and he hit a fair amount of doubles but not a ton, and they didn't turn into HR.  Bottom line, age 25 in Triple-A, he came close to 500 slugging but didn't quite get there, and we are talking about a corner OF.

Taveras earns his spot -- barely -- by virtue of his speed on the bases and range in the field, and he is an elite player in these areas.  Francisco is not even considered an adequate CF, let alone a CF with elite range.  We might be dismissive of Taveras' value if it was only stealing bases -- in fact I am somewhat dismissive of it -- but he does translate it into plus CF defense, which is valuable in and of itself.

As for Barton, at those same levels (A+ and AA), he hit only 30 HR in 294 games, or 15 HR/150.  But unlike the other two, he didn't do it at ages 21-22, he did it at ages 23-25 -- "once he got his man muscles," to use Torii Hunter's memorable phrase.  I won't try to argue that Taveras could have matched those numbers at the same age, but it's possible.  More to the point, Francisco hit considerably better -- 19 HR/150 -- at ages 24-25, and he did it in Triple-A.  And Barton is a little bit too old for us to start guessing that his doubles will become homers, especially as he jumps from Double-A to the majors (or at least the NL Central).

As for speed, Barton stole 80 bases over three full seasons in the minors, 34 SB/150 and he was at the peak ages for speed, 23-25.  Taveras stole 166 bases in his last three minor league seasons, 83 SB/150, and did it at less ideal ages for pure speed, 20-22.  So we're talking about two totally different degrees of speed here, such that whatever edge Barton may have in power -- less than you think -- Taveras surely has that much of an edge in speed.

My point being, Barton very much should be grouped  with Taveras in terms of overall value, and Francisco is definitely a notch higher.  That said, the fact that we can even compare Francisco to the other two tells you something.