不一样。纪念碑是近代才有的，一般用来纪念战争胜利，革命先烈啊什么的。牌坊是古代就有的，在古代一般要得到皇帝的恩赐才可以建牌坊。比如皇帝认为你是一个很有德行的人，或者你曾经立过大功，就可能赐给你一座功德牌坊，以表彰和宣扬你的功德。
bù yīyàng 。jìniànbēi shì jìndài cái yǒu de ，yībān yònglái jìniàn zhànzhēng shènglì ，gémìng xiānliè a shénme de 。páifāng shì gǔdài jiù yǒu de ，zài gǔdài yībān yào dédào huángdì de ēncì cái kěyǐ jiàn páifāng 。bǐrú huángdì rènwéi nǐ shì yī gè hěn yǒu déxíng de rén ，huò zhě nǐ céngjīng lì guo dàgōng ，jiù kěnéng cìgěi nǐ yī zuò gōngdé páifāng ，yǐ biǎozhāng hé xuānyáng nǐ de gōngdé 。
They're not the same. Commemorative monuments are more recent and typically commemorate war victories, revolutionary martyrs and such. Commemorative arches are from ancient times and normally required the emperor's blessing before they could be built. For example, if the emperor thought you to be a very virtuous person or you performed some outstanding service, the emperor could potentially gift you an arch to reward and honor you.