The BoTH is 73 base, the Katana is 74 base. The damages should be approximately the same.
Katana
1.15M ST
Chiko carved into Dropbear Roof Ninja [100032]
Chiko carved into Dropbear Roof Ninja [95132]
Chiko rived Dropbear Roof Ninja [78500]
Chiko dislimbed Dropbear Roof Ninja [82184]
Chiko clove Dropbear Assassin [74955]
Chiko carved into Dropbear Assassin [80772]
Chiko hacked Dropbear Assassin [96766]
Chiko carved into Dropbear Assassin [98288]
Chiko dislimbed Dropbear Assassin [101204]
Chiko carved into Dropbear Roof Ninja [72800]
Chiko carved into Dropbear Roof Ninja [66075]
Chiko clove Dropbear Roof Ninja [100235]
Chiko rived Dropbear Roof Ninja [88027]
Chiko dislimbed Dropbear Roof Ninja [63635]
Chiko dislimbed Dropbear Roof Ninja [95464]
Chiko clove Dropbear Assassin [84628]
Chiko carved into Dropbear Assassin [91356]
Chiko carved into Dropbear Assassin [69243]
Chiko carved into Dropbear Assassin [95380]
BoTH
1.15M ST
Chiko sliced into Dropbear Roof Ninja [66188]
Chiko carved into Dropbear Roof Ninja [92825]
Chiko sliced into Dropbear Roof Ninja [58128]
Chiko carved into Dropbear Roof Ninja [64795]
Chiko eviscerated Dropbear Roof Ninja [57470]
Chiko eviscerated Dropbear Roof Ninja [68268]
Chiko rent Dropbear Assassin [66979]
Chiko chopped Dropbear Assassin [85168]
Chiko chopped Dropbear Assassin [93232]
Chiko carved into Dropbear Assassin [83644]
Chiko eviscerated Dropbear Assassin [65892]
Chiko hacked Dropbear Roof Ninja [71136]
Chiko carved into Dropbear Roof Ninja [78678]
Chiko gashed Dropbear Roof Ninja [73561]
Chiko sliced into Dropbear Roof Ninja [73422]
Chiko eviscerated Dropbear Roof Ninja [90760]
Chiko eviscerated Dropbear Roof Ninja [70744]
Chiko gashed Dropbear Assassin [87769]
Chiko rived Dropbear Assassin [77601]
Chiko carved into Dropbear Assassin [61195]
Chiko carved into Dropbear Assassin [83744]
Chiko eviscerated Dropbear Assassin [80723]
For the Katana:
Max: 101204
Min: 63635
Mean: 86035.6
For the BoTH:
Max: 93232
Min: 57470
Mean: 75087.4
So 1 base damage increase constitutes a 15% damage increase?
Additionally, if you look at my Excel Spreadsheet, the Executioner's Sword hits consistently 10% harder than the Morgul Hammer - but the Exec is 2 base damage LESS than the MH. That makes me think something is awry with the base damage formula for the leech-draining weaponry.
Since this is Changemonth, maybe it would be a good time to see if this is either a bug that needs fixing or we could find out that this is the trade-off we get for our leech - BoTH users actually are swinging a life-leeching THF?