Both，with 马铃薯 being the more formal one (seen mostly on labels or scientific papers) and 土豆 being the less formal one (used in daily conversations, seen on the shop signs.) Although I won't call 土豆 a slang because it still retains a certain degree of formality in itself. There are even less formal options such as 山药蛋 or 洋芋, which is roughly at par with words such as "spud" or "tater" in English.

马铃薯和土豆是同一种蔬菜，土豆是俚语。

马铃薯

都可以