What sort of filling after a root canal filling?

[SixImpossible]

2nd premolar (the one just before the molars is the 2nd, isn't it?), already fairly heavily filled in more than 1/4 of the tooth, including the side that touches the molar.

Dentist says I can choose between a white filling or amalgam after he has completed the root canal work. Function and longevity are more important to me than appearance.

Which should I choose?

My dentist advise white ones where the tooth is heavily filled as they are more flexible so put less pressure on the tooth when you bite.

But OTOH don't white filings wear out faster?

I had a white filling over a root canal on a molar, and it lasted two years. The metal filling that replaced it lasted 20y before having to be replaced by a cap.

White fillings shrink as they are cured and are therefore more prone to leakage. The most important part of root filling is an effective seal in the access cavity ( the bit of the tooth in the mouth rather than the root). So it makes sense to use a material which is less likely to leak. If you are not worried about appearance then amalgam is a better seal. It is also a heavy metal ( large silver content) and bacteria do not like heavy metals so don't tend to grow well around them. They love growing on composite ( white filling).