Everyone may have different opinions for sure, but my personal take on this is that if can afford, don’t take the grant, since if you’re going to upgrade, means you need to return the grant with interest when you’re selling your flat. But if not intending to sell and live forever there, I would take the grant.
If u r eligible for the grant, take it since they will put the money in your CPF account in order for u to purchase ur house.
As for the myth of returning the CPF... there is no such thing because grant = $ put in ur CPF, presume u sell the house, u just need to put the money (the grant amount) back into ur CPF (+interest) and the $ still belongs to u.