Hello! Nothing obvious in terms of a general guideline for ng-if usage seems to pop-up, so general coding guidelines should apply.
Keep in mind the following is subjective, and I think most would agree, but not for sure.
Not a huge deal for simple comparisons like in your example, so normally probably either or is okay.
Exposing variables directly is often seen as anti-encapsulation, so using a controller function instead, especially if the expression gets more complex, would be better.
This is sort of on-topic, sort of not. I don't mind answering if it's fairly introductory like this, and related to our content, but still, better to ask questions like this in an AngularJS forum or stackoverflow/stackexchange.