Its not that 25 ATK is that big of a difference, it's just that the 10 extra crit would be better obtained from an accessory.
I actually came back to this page to reference the stats now that Tactics has returned. (hence the late reply) There is a conflict with what I said in today's current unit building however that only affects players that are using trust stones.
assault is best. you want to recover as much attack as possible. switching from nag or lionheart will drop attack value by ~40-50 for an attack roll (which is compensated for by the lightning damage scaling) and you can thrown on an accessory TMR to add crit.
now if you dont have access to one yet, i would still recommend assault but could see the crit being viable as a short term option. i just would not recommend making two of these as its is looking to be such a niche weapon.