If you can only afford one hearing aid, should you put in the worst ear or the best ear?
I would put in worst ear.
This is a question you should ask your professional.
There are many factors that must be considered, such as the speech discrimination scores. For example, if the worse ear has very low speech discrimination scores, such as 20% or even worse and the better ear with hearing loss has, for example 80% discrimination, then the choice would normally be to fit the better ear.
Thus this should be discussed further with your audi.