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For which simulator do you want to make models?
If I would have to start learning a new tool, I think I would pick Blender today. But if you already know another tool well that might be a good choice as well.
If your focus is primarily scenery, you could also consider to use SketchUp. It has less of a learning curve, but tools like GMax/Blender give you more features when it comes to animations and such.
Arno already said that, my mistake!And if you make a FSX scenery MDL with Blender, ModelConverterX can turn it into a FS2004 model quite easily.
For SketchUp, I don't think polygon count is an issue. It's the modeller that makes the polygons, not the tool. Although it might be easier in SketchUp to waste polygons, it's the choice of the modeller to do so.