I use Runkeeper and Strava intermittently.
Runkeeper: A much more full-featured app. You can turn it on and run, but you can also pre-program intervals and things like that, and it can give you audio cues for the interval transitions. It's a really nice app. Good Apple Watch integration, and can record stationary (treadmill) runs. All data is accessible on their web portal as well. I've been using it for 9 years.
Strava: A very basic app. Missing a lot of running features that other apps have, and it doesn't have Apple Watch integration which is more or less a deal breaker for me. Also can't record treadmill workouts directly, although you can enter them manually. What it does have is a very polished socially media platform. It has clubs, friends, follows, and a really cool "flybys" feature where you can see who passed by you (using Strava) on your runs. It also has some good data navigation features on the web site.
I currently run with runkeeper, then import my data into Strava. I only do this because I am part of a club through my work that organizes on Strava.