As per the title, is it on topic to ask whether a lawyer should recommend the same defence in case X, rather than case Y?
The defence would be a new one, so the first case to use it would be a test case. If the lawyer b believes it would be better tested in case X and then cited as a precedent in case Y , should they attempt to convince the client of that, even insist on it, if the client is opposed?
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.
This sort of question of what someone should do seems likely to be opinion-based (and in fact, the question you asked has now been closed for this reason).
Different lawyers may disagree as to ideal trial strategy. Stack Exchange is poorly suited to opinion-based questions: it's one of the few close reasons that exists on every site. Generally, Stack Exchange questions should be answerable with objective facts, rather than opinions. There should be an objectively correct answer. With a question like this, there isn't.
