As we've discussed previously, core body temperature should be your primary concern in a survival situation. The three primary tools you should immediately turn toin a survival situation are 1) clothing, 2) fire, and 3) shelter. Depending on your advanced preparedness situation, you either will or will not have the ideal clothing with you for your survival circumstances (like going for a day hike in the hot desert but getting lost without clothes needed for a cold desert night). If you do not have the ideal clothing with you, then you can compensate for this with fire and shelter.
Fire or Shelter? Which one first?
Fire and shelter each have their advantages and disadvanges in terms of protecting your precious core body temperature. This is not to say that you should choose one over the other. In most survival situations, fire and shelter work best as a combination as opposed to one being used at the expense of the other. Sometimes your set of circumstances will not allow for both (such as when fuel for fire is not available or shelter making materials are not available), so it is important to learn how to leverage each with mastery.
Let's take a quick look at some of the pros and cons of fire and shelter:
Faster to prepare in a survival situation than a shelter.
Less work to create in a survival situation
Can provide comfort/"companionship"
Can use to boil water, cook, dry gear, and much more!
Fire may be difficult to achieve in certain weather conditions
Fuel for your fire may be in short supply
Fire requires ongoing maintenance
Fire will not protect you from wind or rain
Fire will not protect you from the heat
Fire is more of a short term solution in that if you don't maintain it, it will go out
Shelter can protect you from wind, rain, snow, and sun
Shelter can be created in almost any weather condition
Shelter does not require a constant flow of fuel
Shelter can provide insulation to help you multiply your fire output
Shelter is more of a "permanent" solution/longer term investment
Requires far less upkeep than fire
Can actually protect your fire from wind and rain!
Shelter can take longer to create
Shelter can take more energy to achieve
Shelter does not provide light, ability to boil water, cook, dry gear, etc.
Shelter building materials may be difficult to find
So as you can see, both fire and shelter offer tremendous benefits in a survival situation. Not every survival situation is the same, so your priority will depend on the immediate threats to your core body temperature. For example, if high winds and precipitation are your biggest threat, then your first priority should be taking refuge in either an improvised shelter of some sort or a naturally occuring shelter like a downed tree or rocky overhang. In fact... you probably wouldn't even be able to make a fire out in the open in windy and rainy conditions like that. However, if it is a cold but calm dry evening and you just climbed out of a freezing stream and your clothes and gear are all wet, then fire should be your first priority, then shelter.
In the vast majority of survival situations (not all), you will typically want to get a fire started first and then build your shelter. That way you have a source of heat near by that you can always warm up by as you are building your shelter. This is especially true if you find yourself in this situation at night since fire will provide you light by which you can perform tasks like shelter buidling. But there are occasions when building a shelter should come first. If you find yourself exposed to direct sunlight, rain, snow, or wind and either heat is actually a threat or fire cannot be made until you have a spot protected from wind and wetness, then shelter should be your priority, then fire. This is the kind of prioritization that can mean the difference between life and death for you in a survival scenario.