Only slots above 30 planes can survive E7H, which means Hiryuu K2's second slot and Saratoga's first slot are suitable for a bomber. When it comes to slot distribution, Saratoga has the advantage as first slot gives 10% extra post-cap critical damage. However, after taking her low firepower (11 points below Hiryuu's) into account, her 10% post-cap damage yields worse output than Hiryuu's raw shelling might.
To make it clearer, given Tomonaga as the sole bomber on their biggest slots and all proficiency critical procs.
- Saratoga has [55 + 1.5 x (15 torp + 53 fp)] x 1.2 proficiency crit modifier = 188.4 attack power after crit
- Hiryuu K2 has [55 + 1.5 x (15 torp + 64 fp)] x 1.1 proficiency crit modifier = 190.85 attack power after crit
As you can see, Hiryuu wins the raw output game without the extra 10% from first slot in critical conditions. Without crit, Hiryuu wins hands down.
Other aspects of why Saratoga is a piss poor choice for E7H is her low evasion, which is the lowest among all CV(B) in the game, and slightly higher sortie cost. Hiryuu has more output in all scenario, same number of planes on biggest slot, 54% more evaion and cost much lower to operate.
Take Hiryuu, not Kaga, and definitely not Saratoga