In certain circumstances, they can be. For example, FHA loans cannot be used for the purchase of either a second home or an investment property. Conventional loans can be used for either, in addition to the purchase or refinance of a primary residence.
Conventional loans are also the better choice if the mortgage you want is 80% or less of the purchase price for the value of the home. That's because conventional loans don't require mortgage insurance on mortgages of 80% or less of the property value, while FHA loans do.