It's a hard one to answer, because both of them have so many different qualities, depends mainly on your body type, injuries and preferences:-
Running
Running is such a great way of losing calories, you can burn up to a 1,000 in an hour, depending on your weight and speed, its one of the fastest ways to burn calories
612 Calories burnt per hour (based on 90kg man running 5mph)
Relatively low cost, all you need is a pair of running shoes and shorts
You can run anytime, anywhere and in any country
Great for getting out and about and exploring local areas
Running releases endorphins so great for the head and thinking time to release stress
Not great for joints and your lower back as we grow older
Swimming
Great on joints and muscle, very low impact
Good way to improve and increase lung capacity
476 Calories burnt per hour (based on a light/moderate swim, 90kg man)
Need a swimming pool, lake or sea - so an element of travel is required
Low cost as the only equipment required trunks, goggles and swim hat (optional)
Being a confident swimmer is a great life skill
Improving your swim stroke is key
Can make you feel hungry afterwards
Repetitive if you are in a pool
Summary
Running is such an accessible sport to do, as it's on your doorstep. However, if you have any joint issues or injuries then swimming is the better one to do and may even help you rehabilitate your injury, so you can then run a little.
Brett's view: Running has enabled me to stay moderately healthy and fit, whilst at the same time seeing some fantastic cities and countryside by foot.
David's view: I am really enjoying swimming 3 times a week in a heated outdoor municipal pool. It provides me with a stress free cardio workout, which I miss from being unable to do too much running due to a Rugby knee. I am working on my stroke and speed. For info on outdoor pools https://www.outdoorswimmingsociety.com/uk-winter-lido-list/