Very risky trade, but I think I'd do it. You'd be selling high on two stars in the making, which is a tough trigger to pull. Plus, it's a very even challenge trade. The drop from Weaver to Cahill is on the same level as Tulo to Castro. But, in an OPS league, Tulo is HUGE at SS, with a decent shot to be the #1 player. The key is Cahill's K/9 rate. He showed a good one in the minors, a "meh" one last year, and has had games this year that make it seem like he could put up an ~8 K/9. If he does that, you probably win this trade.
As it is, you stand a much better chance filling out your rotation on the WW than you do your SS position. There are no Tulos on the WW.