Citrus fruits in general are hard on the liver and spleen, and that includes lemon juice. Lemon juice is better than orange because it's soooo hard to drink as much! LOL! ;)
Plus lemon really helps digestion in OTHER ways, such as helping the stomach to produce more HCL. Orange juice is actually a culprit in getting too mucosy - blech! ;)
So pick lemon over orange but don't over do it.
Apple juice, and to a certain extent, cherries, berries and green grapes, are fanastic juices, very balanced, not stressful to the liver. When in doubt, go for apple juice!
Hope that answers your question! :)