Is it on topic to ask whether a lawyer should recommend the same defence in case X, rather than case Y?
That is on topic insofar as assessing a litigation strategy serves educational purposes (see the disclaimer at the upper right portion of every Law SE post). It is noteworthy that keeping the post in overly generic terms is unlikely to be of any interest. The post would need details that are relevant enough to prompt the audience to think why the answer might differ from the general conclusion.
Generally speaking, the principle of "equal protection of the laws" implies that choosing case X over Y as potential precedent is inconsequential. Furthermore, the strategy or choice would be unavailing if Y is distinguishable from X.
should they attempt to convince the client of that, even insist on it, if the client is opposed?
That is unanswerable without knowing enough details of the case, including the lawyer's rationale as well as the client's reasons for rejecting the lawyer's proposed strategy.