My understanding of it is that dom-if does not stamp until the expression becomes truthy, but after doing so it behaves much like [hidden] does. In that way dom-if is a pessimistic [hidden] that defers stamping for as long as possible.
This lazy-loading approach is favorable in certain situations where stamping the template would be very resource intensive. For example if the template was very large with multiple custom components that have to be initialized. If you had just used the hidden attribute there you would pay the performance cost of creating all of that DOM only for it to not be visible until later.
For simple cases, such as hiding or showing some text, a div or two, etc. the hidden attribute may be faster because the cost of creating those elements and then hiding them may be less than creating a <template> instance to hold your code, Polymer setting up listeners to monitor the expression for truthiness, then, when it becomes truthy, all the overhead for stamping the template, parsing it for binding expressions, etc. This is particularly true for the browsers where <template> support is polyfilled.
Thinking it through for your situation (and ideally testing it) is the best route to take. Often times the differences may be negligible but do take special care if this part of your code happens to be in a dom-repeat with a lot of items or anything that happens frequently. The difference could add up.
