if same locale, all factors being the same, surely the 15yrs FH. Unfortunately, no development have same factors because they are not sitting on same piece of land, and developer is different, so many things to consider..., since both have already TOP for sometimes, market forces would have dictate their value. So u can easily check their average transaction psf over last 6mths, which give u quite a fair indication of what value market perceive unless there are no transactions in this period.
U are asking hypothetical questions, not sure if any of our answers will be able to satisfy the answers you are looking for.