White fillings are they preferable to amalgam ones?

You either trust your dentist to do the right thing or you don't. If you don't then you need a new one. There must be a reason the doc is saying the only choice is a composite. Amalgams can be place in most any situation [they were used way before composites were developed] and NO, they don't cause any more problems than composites do.

A lot of dentists prefer the white fillings now days. Amalgam isn't really dangerous as we all hear, but for a long time was the strongest choice. Now that there are many advances in the dental products a lot of the composite (white) fillings are just as strong and durable.

Also amalgam fillings tend to expand over the years leaving teeth cracked and needing crowns.

I would go with whatever your dentist recommends because there is most likely a reason that he likes to use that particular material.

The reason why the dentist only choose white filling material, maybe due to a small fractured. Amalgam needs an undercut type of cavity to retain it inside the cavity because it not adheres to the tooth surface. So, more tooth structure to be removing to create the undercut even thought it sounds. But, by using white filling (composite), it can adhere to the tooth surfaces, so no undercut being created. Meaning, less tooth destruction.

I would say that they are preferable. They not only blend in with the tooth, but many dentists are wary of using amalgam because it contains metal (many often contained mercury). White fillings also cost less.