难 is difficult 累 is tired the meaning of the sentence can be understood by chinese, but the structure is a little strange. beacuse the concept of these two words have little different， and the adj of your sentence “上山a还是下山b” always have to be the same. 是上山难还是下山难? 是上山累还是下山累? 是汉堡好吃还是披萨好吃？ its the structure to compareing two similar things， so the things your compared need to be the same. just like “是汉堡好吃(delicious)还是披萨好闻(smell good)?” its kind of strange.

Thanks

这个很对