New floorig onto joists or overlay onto existing baltic pine?

Hi,

I am new to weatherboard houses on stumps, coming from brick and tile on concrete in Perth, and have found this forum really useful to increase my understanding. I am now getting conflicting advice on replacing our floor by nailing to joists or by overlaying what is there and would appreciate advice.

We have had to go through our exisiting baltic pine floor to restump, and have had a couple of quotes on patching up versus replacing the whole thing with a hardwood (eg a remilled messmate blend). In the long run I'd prefer a hardwood floor so we will probably do that. But one guy has said we should rip up the remaining floor and secret nail the new floor straight to joists, and that there is no need for any kind of insulation under the floor as it doesn't make much difference. Another has said we are better overlaying the new floor (still proper timber) as it makes it more stable and provides insulation. They would screw plywood to the baltic pine then glue the new timber.

To what extent do more layers on the floor improve insulation? Is it a noticeable difference? Are there problems with overlaying? (can the wood "breathe" properly? What if something happens to the timber underneath?) Does it make a difference if the new floor is a remilled or a new plantation timber?

Thanks for any advice.