What Akainu has done:
-Actually battled.
-Killed "Fire Fist" Ace.
-Injured/killed multiple people.
-Took 2 hits from Whitebeard, and came out of it basically saying "NOW WHERE WAS I?", proving he has ZWOMFG crazy stamina.
-Gave Whitebeard multiple fatal wounds.
-Deceived a pirate to badly injure Whitebeard.
-Leading cause behind Whitebeard's death. He didn't directly kill him, but he badly injured Whitebeard to the extent Whitebeard couldn't take on Blackbeard pirates.
Things Sengoku has done:
-Give a few orders.
-Give Luffy a minor injury.
Also, he allowed Aces escape and he pretty much has done no fighting.
Adding to it, Akainu has a godly DF and is easily the most powerful marine from what we've seen. Also, Absolute Justice. He's VERY serious about it. He would get the marines off of their Lazy asses, and they'd actually be powerful.