兩個都常聽見。在台灣的話，用「哪裡」的人比較多數。例如：「你剛才去哪裡？」

just like a friend says,哪儿 is used more frequently in north and less in south,because in south people don't say "儿化音“ . the "儿化音” is kinda like "Lily"~"Lill" ,and it usually used in casual occasion a spoken language. hope this can help

哪兒 is more nort east. (ex, beijing) 哪裡 is use more in south.

“哪儿” is more spoken,especially for people who live in Nord, （cuz southern don't always use “儿化音”（←do u know it?

哪里。