Good question, but which alternative is correct depends on how you define platform. If you define "platform" as "versions of SQL Server", then CONVERT works in every version whereas CAST...

Oh, I know that. From the perspective of someone who started with SQL Server way back in 91 it's completely clear. But for the less experienced it can certainly appear...

I got it right because I knew it, but I understand why that might not be obvious, and the behaviour is slightly iffy.

If you do

SELECT @var = MAX(object_id)

FROM...

This used to be true, and I have often used this knowledge to optimise slow-running queries. But since SQL 2005 it no longer matters from a performance point of view....