Actually it depends on what are your initial materials (reactants) and what is their melting temperature.

For Sol-Gel method it is better to use "Nitrate" form of your reactant like "Bi(NO3)3.5H2O" for Bismuth , moreover "Nitrate" form of materials is more expensive and poisonous  and less staible in high temperature. But if you want to use Solid state reaction method then you should use the "Oxide" form of the materials like "Bi2O3" for Bismuth. usually Sol-gel method give more homogeneous product (less impurities).

For the preparation of double perovskite the better method is sol-gel method because of its of its homogeneity capability. for the sol-gel methods nitrate salts are better as I am also going to do with these and also check the result for the rough analysis Its quite satisfactory some unwanted phase has observed but try to remove these now.

Depends on your target material. As all nitrates are not easily available one can also prepare using solid state reaction, however, some precautions like determination of calcination and sintering temperature, homogeneous mixing etc.

Solgel method is good one

solgel method especially Pichini Method