After reading from this link, it makes more sense:
'Would' is used in a case where it's already been established that something in the past is being referred to. For example:
When I was younger, I would bike to school.
This sounds natural as opposed to saying something like
When I was younger, I used to bike to school.
In contrast, using 'used to' sounds more appropriate here:
I used to bike to school every day when I was younger.
Combining the two:
I used to bike to school every day when I was younger. I would meet my friends after school to go to the park.
The link probably explains it more than I can!
Edit: I've researched a little more and am not able to find the best of answers, but I did find a few more examples. I think the proper use of 'would' and 'used to' become more natural as you learn more English.
Example from 5minuteenglish.com with emphasis added -
"Used as a verb. Used to + verb is a regular verb and means something that happened but doesn't happen any more. It uses -ed to show past tense. But since it always means something that happened in the past, it should always use past tense.
Ex. "I used to go to school in Paris. (I went to school there before, but now I don't.)"
Ex. "When Joshua was a child, he used to climb trees. (Now he doesn't climb trees.)"
Also, by the examples from this site I would say that would is used sometimes in more of a hypothetical 'what if' type of sentence.