Buster up VS NP damage up.

karna has %30buster and %20 np damage up on his second skill when maxed right? he gets the same %50 np damage boost arturia gets on her 2nd skill %50buster buff. but karna has the disadvantage of getting %20 less damage on his buster cards that turn so arturia's skill is more effective in general. but the developers gave karna that skill to even out his overloaded atk stat and third skill.
long story short. yes if the percantage is the same BUT if it were an arts or quick card effectiveness for arts of quick np,it would be better if you chose %30 quick or arts effectiveness over 15%quick or arts effectiveness+15% np damage

Iskandar is correct that Buster Up applies to all Buster cards, including Buster NPs. However, there is some additional nuance because buffs of the same type stack additively whereas different buff types stack multiplicatively. So, Artoria with Partake with the King (+10% Buster) and Mana Burst (+50% Buster) gets +60% Buster for all of her Buster cards, but Artoria with Mana Burst and a +10% ATK CE actually gets 65% damage up on her cards (1.1 * 1.5).

In reality, most of the time multiplicative vs. additive buff stacking isn't that important. However, it can make a difference when you have servants that can put together very impressive buffing percentages. A level 10 skills team of Artoria Archer with Heaven's Feel (+40% NP damage), Waver, and Tamamo gives +80% Arts, +48% Attack, and +40% NP damage. Those buffs multiplicatively stack and result in Artoria's NP dealing ~3.73x its normal damage; if those buffs were all of the same type, it would be only 2.68x NP damage. Shift from Heaven's Feel to Black Grail -- which "only" has 20% more NP damage up than Heaven's Feel -- and you're now dealing ~4.26x NP damage. So, that's why a lot of players will talk about completing the "holy trinity" of offensive buffs, i.e. attack up, card performance up, and NP damage up.