This is one of those hypothetical questions I do not mind answering.
If I had to choose between two exercises or two methods of training – What would I choose?
This comes from a young man trying to figure out what he can do with the least amount of equipment as he prepares for a military career after high school. I am going to answer this question as if I were you first, then will make my recommendation as an aging former military member trying to be "all I used to be."
As you prepare for military service, I would focus on calisthenics and running and rucking. Getting used to running and higher repetitions of all calisthenics (pullups, pushups, situps, other abs, plank poses, squat, lunges, etc) will help with the muscle stamina you need for the daily grind and PT tests. Mixing in some dumbbell work to build the back, shoulders, legs, and arms is not a bad idea and is relatively inexpensive and easy to find.
As an aging athlete, I know we all are a bit different with our top two exercise preferences, but you will not get much argument from many as a way to stay in shape will be mastering the pull-up and the non-impact joy of swimming. Pulling your own weight is an important skill and the non-impact, full-body resistance and cardio effects of swimming are great methods to stay in shape. But to be honest, I do not think I could limit myself. You also need to lift some weights and focus on mobility (maybe add Yoga). As I continue to age, and when my joints do not favor running anymore, I will gravitate toward yoga and swimming.
For the future military member (Tactical Athlete), one thing you have to realize is that you need to be good at everything; being great at any one element of fitness is not needed. Some athletes do not need to even run but need to have great water skills and endurance (swimmers) and others need to be strong as an ox with speed and power and agility with little need for long distance endurance (football players). This is what makes the athlete and the Tactical Athlete different. A variety of running, swimming, rucking, lifting weights, high rep calisthenics and mobility drills offer improvements to the fitness elements of endurance, muscle stamina, water and land skills, strength and power, and flexibility that the military member needs.
Stew Smith works as a presenter / editorial board with the Tactical Strength and Conditioning program of the National Strength and Conditioning Association and is Certified Strength and Conditioning Specialist (CSCS). There are also over 800 articles on Military.com Fitness Forum focusing on a variety of fitness, nutritional, and tactical issues military members face throughout their career.