Philosophically and ideally, these two composer may have explored similar territories. But technically, I find them very different. I mean, I don't find too much in common as far as the orchestration, polyphony and even harmony are concerned. If I may, I think Mahler was the better orchestrator of the two and his use of polyphony is second to none. Because of this Mahler's music oftentimes sounds more modern and sophisticated than Shosy's (think of M9th or M10th). Having said that, I do think Shosy was better at expressing darker sides of human nature; he delved deeper into the meaning of despair, depression, violence, horror, solitude than Mahler did. Furthermore, Shosy was more direct and as a result audiences may find his music more reachable and powerful.
Bottom line: although there may be traces of similarities between the two composer, I think they are different in their nature.
John,