Welcome to the site Isag.
We are going to have to get some more detailed info on your floor structure first as you mentioned you are thinking about installing marble.
The deflection requirements for a stone floor are almost twice as much as those for a regular ceramic/porcelain tile floor.
Can you tell us what the joist material is, including the joist spacing and the size of the lumber and the span?

As for the radiant heating, whether it's under the subfloor or on top is not going to matter too much as far as heating the floor and room goes. I have done them both ways and they both work fine.
Obvious reason for under the the floor has to do with height.
For a top mounted hydronic system you need to decide what type of material your going to use to cover the pex tubing.
For a gypcrete or cement based pour the overall height is a min. 1-1/4" to 1-1/2" from the top of the subfloor.
Another product to consider is Schluter Bekotec which is a modular screed system that uses drypac mortar to embed the pex. Check out the web site. http://www.schluter.com/9_1_schluter_bekotec.aspx
It has a definite advantage in reducing the total dead weight and amount of mudbed material used. Drawback is the overall height with Ditra is almost 1-3/4" to 1-7/8" plus tile height.

Is there any particular reason why you want hydronic instead of an electric wire type system?

The bath is standard from floor perspective. Joists are double or triple 2 X 8 dimensional lumber. Subfloor is ¾” plywood or OSB glued and nailed to joists.

But what do you actually have? What's the span of your joists?
Are you saying that the joists have already been sistered?

You keep asking about using marble, but it must first be established whether your floor structure will be sufficient to even install stone,irrespective of the floor heating.

We can't be of any help if you can not give details regarding the current floor system.