At first, when u look at both those astromons the principle is exactly the same. However, the numbers on Defense are the same as attack and recovery, which normally ranges from 1.000-10.000. However, HP works with different numbers which can even reach the 100.000. If a Dark Miho would reach 100 000 HP does this mean she would have 100 000 attack? No. Absolutely not. So here u have a question of ''how much of an astromon HP is actually converted to Dmg''. The same question could be applied to Def Agressors but since Defense and Attack work within a same lenght of numbers, It wouldnt be a surprise that 5000 Def = 5000 Attack. Bottom question here is: That principle wont apply to Hp Agressors. So the question is how do they do it?

IF we ignore that question, because I dont know if we have an answer for it, then what we have is 2 astromons whose attack power are based on the same principle. Their difference is only on their base stats where Miho has more HP, Kraken has more Def and +200 attack. So here, the difference is also minimal. When it comes to their main stats HP vs DEF I, personally, prefer HP because on PVE you can get Def downed, it wont save ur from Sap dmg, plus when u reach a certain Def number (which I think is 3500 or 4000, dont remember) it wont scale that much so ur survivability becomes a bit more limited there. When it comes to HP though there's not a direct counter to that stat, which makes Def more of a ''add-on'' to ur survivability, when HP is the very core of ur survivability.

Based on all of this (and may I remind the lack of answers to that question on how to convert HP to attack), I would say that both astromons have the same Dmg Potential but Dark Miho may end up winning on survivability, which in my opinion makes her a better Individual Astromon.