If you do not know which one to use, remember that used to is always correct. So, it will be better to choose used to in case of any doubt. But if you want to move a little further, you should start using would whenever possible. It will make you sound much better and show greater fluency in English.

Unlike used to, would  is only correct if the described action is a repeated action over a period of time:

• As a child, I would walk to school every morning. 

• I would drink two glasses of apple juice a day back then.

• Every evening, when I got home, I would leave my shoes at the door.

It cannot be used for more passive, permanent or part of a person's lifestyle actions:

• I used to have two dogs, but one of them died last month. 

WOULD repeated actions / habits                                                                                                              USED TO   repeated activities in the past / states / situations that don't exist now

Not for: (((+))) Subject + USED TO + verb + complements.

• states / passive actions (((-))) Subject + DIDN'T USE TO + verb + complements.

• permanent actions (((?))) DID + Subject + USE TO + verb + complements.

• lifestyle 

• professions / education / learn a new skill

EXAMPLES:

• Have lots of freckles. — Only used to

• Dream about kittens every night. — Used to or would

• Play with the children in the neighbourhood. — Used to or would

• Exercise at least once a day. — Used to or would

• Work as a journalist. — Only used to