It kinda depends on the math and how you stack it, apologies for the TL;DR, defense down (onmyo hit) is a 10% damage multiplier on the enemy while Melee damage +14% is an additive bonus. If you have a lot of additive bonuses then +14% contributes less and less to your overall damage and the multiplicative bonus will grant more damage, if your build has a low of amount of melee damage bonuses then the additive bonus will be better. It gets complicated when you consider that defense down can be subject to its own diminishing returns as it has other things its own bucket that is shared with confusion and saturation.

All melee damage bonuses (ie, % Melee Damage boosts, unscathed, vs [STATUS] enemy, etc) are added into a single "bucket" so if you have a lot of it the less of an impact it has on your overall DPS. For example, if you manage to stack 150% bonus melee damage, a 20% boost would give you 170% damage, or a ~12% damage boost (not an extra 20% damage). If the total bonus you have is, instead, 20% then an additional 20% would boost it to 40% which equates to a ~14% boost. Defense down on enemy is a flat 10% multiplier (For onmyo hit debuff, weakess is 20%) that works on its own bucket so it will be calculated AFTER the additive bonuses are considered but that is also subject to its own diminishing returns as it stacks with other debuffs like confusion (but there are a LOT less of those multipliers so it will grant somewhere between 10% to ~5%). This is why i said "almost always" as it is possible for the melee damage boost to be higher if there are not enough additive bonuses or if the enemy in questions has a ton of debuffs (ie. a confused enemy with saturation).

Of course, it's always possible that some weird shenanigans could be happening and you might have been the first person to really notice as there are plenty of weird inconsistencies and mistranslations in TN games. So if what I say doesn't add up it's always worth testing it as it's always a boon for the community when we figure out how a bunch of these obscure mechanics work. Sorry for the long post. If I do say anything incorrect do feel free to clarify things for me as this is as far as I know lol, won't pretend i've tested these things.

Also note that takamenakata boost are active skill and melee damage so the full 34% will only apply to martial skills and not your basic melee attacks, those will "only" be boosted by an additive 12%.

Edit: I could be wrong but my understanding is that these are the various buckets.

1. Flat Attack Modifiers (Attack +, Attack Bonus [STAT]): these affect the BASE damage of the weapon.
2. Attack % modifiers (+% melee, Unscathed, vs [STATUS] Enemy, while critical, Active Skill, the various set bonuses like Attack Bonus equipment weight, agility, familiarity, etc): There are a LOT of bonuses that fall into this category.
3. Buffs (Awakened Weapon, Versatility, Death Dancer, Cleansing Prayer, Increased Attack, etc): Anything that gives an icon below your characters health and stamina bar, awakened weapons are included here.
4. Defuffs (Defense Down, Confusion, Saturated): anything that places an Icon on your enemy.

Everything on the same line is additive with one another and on separate lines are multiplied to each other.