I think you mean the difference used to and would when both they refer to past habits . A matter of frequency...
There are some slight differences :In a text used to enables the speaker, the utterer, the writer to set the situation in the past, to build a frame for his narrative. When I was a little boy, there used to be a church. He used to run every day : Used to marks a statement of facts about a past habit -rather coldly without any commitment from the speaker.
Would shows that the speaker attributes a past behaviour, a habit to someone. The focus is on the person on whom the speaker comments.
“used to” is used to describe past habits or long lasting actions and situations which are now finished.
“People used to think the sun is revolving around the earth.”
For questions and negative forms, two forms of the verbs are used –either the normal infinitive pattern after “did” (more common) or the past form “used” (less common).
“When you were a kid, did you use to think the sun revolved around the earth?”
“I didn’t used to take such a large dress size, but now I do.”
In a more informal style. Questions and negative are possible without “do”, although these forms are less often used.
“I used not to like contemporary dance, but now I do.”
“Used you to play the organ in church before you become a monk””
When we used “used to” we are describing things that happened at an earlier stage in our live which are no longer in place as circumstances have changed.
When we telling a story and recollecting an event from long ago we often prefer to use “would” to describe repeated behavior in the past, although both “would” and “used to” are possible.
“Do you remember what we used to get up to when we were teenagers? How I would wait for you nearly every afternoon after school and then we would stroll home together across the park, holding hands, and you would feed the ducks on the pond while I had a cigarette?”
Note, that “would” in this sense describes past events and actions. It cannot be used to refer to past states. To describe past states we can only use “used to”.
“I used to live in that home over there.”
(Not: I would live in that home over there.)
“I used to own a 1966 Silver Cloud Rolls Royce.”
“I used sometimes to drive to work in it.”
“I would sometimes drive to work in it.”
“When he was at university, he would sleep until noon at the weekends.”
“My daughter lived in Vienne for many years, but she would always come home for Christmas.”
I would always forget my homework, until the teacher threatened to punish me.”
“Sleeping until noon”, “t the weekends”, “comes home for Christmas”, ”forgetting my homework” all these things were repeated many times, again and again and again.
This is difference between “would” and “used to”. We only use “would” to describe action or situations that were repeated again and again and again. But we use “used to” for any extended action or situation in the past.
“I used to live in Sofia but I moved to Varna last year.”
Can we use “would” instead of “used to” here?
No we can’t, because “living in Sofia” wasn’t repeated again and again and again. It is only a situation in the past therefore only “used to” is good in this sentence.
The second example again:
“When he was at school, he used to paly football every Saturday.”
Here we are talking about “playing football every Saturday.”This is an action that was repeated many times, so we can also say
When he was at school, he would play football every Saturday.”
“Used to” and “would” are both good here, and the meaning is the same.
Finally the third example, again
“Did she hate her job many times?”
“She used to hate her job until she got a promotion.”
This isn’t a repeated action so in this example “would” isn’t possible, we have to use “used to”.
USED TO is not a modal , would is. Their respective nature give them different values.
I used sometimes to drive to work in it. I would sometimes drive to work in it.
When he was at school, he used to play football every saturday When he was at school, he would play football every saturday.
In those examples,USED TO simply locates a repeated event in the past with no link to the situation of utterance ( aoristic location). The assertive modal manner prevails here, that is a mere statement of facts, a mere piece of information.
WOULD gives off a different effect: There is a characterisation of the subject 'he' or 'I' in the examples. The utterer comments upon the past characteristic of a person or a character :that was HIS habit to play football.= knowing him it was easy to predict that he / play/ed football = That was how he was.
WOULD is thus used in narratives as a descriptive device to build a character, to focus on him . ( cf Weinrich's time and tenses). Frequency and repetition are not the most important things : characterisation counts above all.
As it was well said, used to is compulsory for an unique event which lasted or occured in the past.
yet, we can still find would with BE :
When I was a little boy , there would be a church here* is incorrect
In all the villages they built in those days, there would be a church in the middle is possible.