So going by this SBS (nice dig) if Kizaru and Akainu fought it would be even tougher for Akainu (there is no mention though of Akainu being stronger than Kizaru except that Akainu was promoted to Fleet Admiral). Maybe Akainu is more ambitious and Kizaru is lazy and all that and that's why he was not considered for that position like Akainu and Aokiji. So our OG admirals are ranked like Akainu > = Kizaru > Aokiji (since Akainu beat Aokiji and Oda basically says that Akainu and Kizaru are stronger than Aokiji) and if it took Akainu, who does have a very ruthless personality, 10 days to defeat Aokiji and split the island in equal halves than the gap between Akainu and Kizaru is even less almost minuscule.
I expect Akainu to be stronger when Luffy eventually fights him.