I use both. My wife and I use the "live tracking" on MMR to keep track of each other when we ride/walk/hike alone. That's the primary difference, particularly since Strava upgraded their route planning/mapping features. But the Strava user base is larger and more competitive, and there are more Strava clubs in the areas where I ride, so I use Strava to track my progress on segments, fuel my need for competition, and analyze heart rate and cadence.

Once nice feature that MMR has is food and weight tracking. Had I known about it when I started my weight loss program, I would have used it as a one-app solution.

BB