I think you are referring to the following kind of expressions:
"I would often go swimming"
"I often used to go swimming."
There is probably a formal grammatical explanation for this. However, my own instinctive sense is that their meanings are the same and the difference in their usage is more a question of style.
For example, you could say this:
"I used to go shopping in street markets. I would often buy fruit and vegetables. Then I would stop for a coffee with a a friend etc"
For the first expression, you could say "would" but you would have to add "an expression of time such as "in the past" to make the meaning clear.
In the second and third expressions, if you said "used to" instead of would, this would sound a bit unnatural.
So, you could say that "used to" sounds more natural when relating to a general comment about the past. "Would" sounds more natural to express more particular activities in the past, to help bring the listener back to that time with you.
I hope that makes sense - it's not a classical explanation and it may not be comprehensive but I hope it helps. Michael