söker is mostly used with abstract objects, if you use it with concrete things it sounds more formal letar works for both and sounds less formal with abstract things.
I don't know much of anything about it, so don't take my word for it;
But I think that "leta" might be used when one is searching for an object.
" Han letar efter nya offer."
I think that "söka" can be used when one is searching for something inevitable.
"De sökar efter lycka."