Both are used to describe something done habitually in the past. In that context, they're almost completely equal. For example: "I lived next to McDonald's for a year. I would go there everyday to eat a hash brown." "I lived next to McDonald's for a year. I used to go there everyday to eat a hash brown." However, "used to" can also be used to describe things that we're done continuously. For example: "I used to live next to McDonald's." "Would" of course is also used for conterfactuals. For example: "I would babysit your dog if I weren't allergic to dogs."

If you are talking about habits, activities and occurrences in the past, you can use either, for example: When I was a child I would spend hours reading fantasy stories. When I was a child I used to spend hours reading fantasy stories. The only difference between these sentences is that the 'would' form is slightly more formal and literary-sounding. Note that you can NOT use 'would' to describe situations and states. For example: When I was a child.. I used to be shy. I used to live in London. I used to have blonde hair. For these situations, you can only use 'used to' - never 'would'. 'Would' is only used for something that you used to 'do'. If in doubt - always use 'used to'.