What is the difference between `while select` and `QueryRun` in X++?

Use while select when the query is known at compile time and the structure is straightforward.

Use QueryRun when you need to build or change the query dynamically.

Query query = new Query();
QueryBuildDataSource qbds = query.addDataSource(tableNum(CustTable));
QueryRun queryRun = new QueryRun(query);

while (queryRun.next())
{
    CustTable custTable = queryRun.get(tableNum(CustTable));
}

If the query is static, I normally keep it simple and use while select. If the user or runtime logic is changing datasources, ranges, or joins, QueryRun makes more sense.