which is the best option for me?

[primal willow]

I would like to create models for both game engines but which can also be used for Minecraft (mods and textures) could you tell me which option I have to select to create and which format should I use for each single case mentioned above?

[quasi berry]

both editions use very different formats

there is no format that works on both

unless you meant something else by "game engines" than Java and Bedrock

[primal willow]

i mean blender, unity ect.

sorry for bad english

[quasi berry]

for blender, unity etc. the "Generic Model" format would be best (save it as bbmodel if you wanna edit it later and export as fbx or obj for blender etc.)
you can use obj model in mods too (at least for Forge) like this:

!faq forge-obj

[pseudo summit]

Forge does allow you to use .obj models in Minecraft, without making a mod.
Here is a step by step guide how to use that feature.
1. Flip your model along the z axis, south and north are flipped in the exported obj models.
2. Move your model by +8 on the x and z axis.
3. Each cube and mesh, must have a unique name
4. You can export the model now, default naming rules apply
5. You'll have to edit the .mtl file, it has paths to your textures and those need to be adjusted for MC resourcpacks.
Here is an example, map_Kd example.png it is recommended to replace that with map_Kd #example.
The # defines it as a variable and simplifies things a bit.
6. You'll have to create a JSON file for your model, with following content:

{
    "loader": "forge:obj",
    "model": "<path to your obj model>.obj",
    "flip-v": true,
    "textures": {
        "particle": "<path to particle texture>",
        "example": "<path of the another texture>"
    }
}

The example in the textures here, is the replacement for the variable we defined in step 5.
This makes it possible to use this .obj model for various different blocks now as you'd do with normal models.

That is basically it, it's not too complex once you get the hang of it.
You can also define display settings in the json file.

[primal willow]

if I just want to make a model to give a friend of mine to create a mod, is it not enough to give him the model in .obj?

[quasi berry]

no

if they want a model for a mod then it's best if you give them a .bbmodel file of a model made in the "Modded Entity" format (or Block/Item format if it's for a block or item)

They can then export it the way they need it.
While converting between formats is possible different formats still have different restrictions that get applied during conversion, hence why I would not suggest it.

If they wanna do the forge obj method you could do the .obj thing, but then you need to send them texture and .mtl files separately too.
A .bbmodel project file will contain everything.