Art Card effectiveness or NP Gain Increase

Art cards effectiveness Increases damage, and arts gain of only arts card while NP gain increases NP gain of each cards
Meaning for damage Arts effectiveness and for NP gain and faster NP then NP gain increase.

I believe arts effectiveness increases damage as well as np gain, whereas np gain just increases the amount of np gauge you get when using arts cards.

Arts card effectiveness = +arts cards damage, +arts card np gain, not rly sure about arts cards star gen

NP gain increase = np gain of all your cards that DOES give np. like quick arts and extra or buster if Arts is the first card in the chain.

You're prolly asking which is better. Now, it depends on the servant using it. I believe NP gain increase does give more np gain than Arts card effectiveness. So for servants like Hans and Tamamo(arguably) who you like to be spamming their NP, it's better off to give them NP gain increase coz you need them to be spamming their NP more than dealing stronger damage. For servants like Artoria(Archer) which is an NP spammer. NP gain increase can help more with NP spamming but in this case many can argue that Arts Card effectiveness is better for overall damage output and NP gain coz she is supposed to be a DPS servant.

The main difference is in the damage as they said. Arts effectiveness only affects arts cards and is beat used on servants with multiple arts card preferably also including an arts np that does damage. Np gain CEs on the other hand increases all np gain. So any card that can get np (even buster cards can gain some np if an arts card is the first in the chain) as well as any enemy hits on your servant. Overall they are better for np gain but don't boost any damage at all. So the preferable users of these are support units that you want to np often. Tamamo could be one for example although I will say majority of supports are better used as holders of CEs that provide benefits to the rest of the group (like fragment of 2030 to get crit stars) or benefits to you in the form of extra qp or extra FP or extra bond points.