Steelmanning is about fixing errors in an argument (or otherwise improving it), while retaining (some of) the argument's assumptions. As a result, the argument becomes better, even if you disagree with some of the assumptions. The conclusion of the argument may change as a result, what's fixed about the conclusion is only the question that it needs to clarify. Devil's advocacy is about finding arguments for a given conclusion, including fallacious but convincing ones.

So the difference is in the direction of reasoning and intent regarding epistemic hygiene. Steelmanning starts from (somewhat) fixed assumptions and looks for more robust arguments following from them that would address a given question (careful hypothetical reasoning), while devil's advocacy starts from a fixed conclusion (not just a fixed question that the conclusion would judge) and looks for convincing arguments leading to it (rationalization with allowed use of dark arts).

A bad aspect of a steelmanned argument is that it can be useless: if you don't accept the assumptions, there is often little point in investigating their implications. A bad aspect of a devil's advocate's argument is that it may be misleading, acting as filtered evidence for the chosen conclusion. In this sense, devil's advocates exercise the skill of coming up with misleading arguments, which might be bad for their ability to reason carefully in other situations.