That would depend to a large extent upon what you intend to do with the model. If it's going to sit on a shelf for display, I'd just fill it with sand since there'd be no need for something solid inside. Other than that, the purpose would determine what sort of weight you want. The choice of material should be based upon specific gravity rather than general physical properties. Cement will do the same job as silicone, but it will be a lot heavier. There's also a whole field of binary materials such as automotive body filler, acrylic "paperweight" liquid, Jell-O (think Gummy Worm concentrations), JB Weld, etc..
What do you want the end result to be?