Comparing the two starters, Cavalli averages only 4.1 IP/GS while Alcantara has been going 6 and a half innings per start.
Cavalli has avoided HRs to an unusual extent this year. Among pitchers with 30+ IP, he's 5th lowest, behind Wrobleskii, Fried, Lugo, and Schlittler. His BABIP is the worst. Literally.
Alcantara has a better than average luck on balls in play, and has suppressed homers somewhat, but both starters have comparable LOB%. Cavalli's big edge is on Ks.
Whiie they take different approaches to it, what you end up with is comparable ERAs for both starters (4.15 Cavalli, 4.01 Alcantara, with a flip on FIP).