I started on MapMyRide because it was really easy to map stuff. In fact I've been using map my run for a year or two longer than I've been back on the bike. I like how the mapping side works, it's easy to use once you are used to it and has a reasonable AI for following roads and cycle paths. It's still my go to mapping tool and it synch's really well with the Garmin to upload rides. The hills aren't categorised the same way as Strava and seem to be more generous than Strava (also they have a Cat 5 classification all of their own.) It will tell you how well you're doing vs yourself and others, although there are less people on it than Strava. I find it a lot easier to map out the rides that I didn't have my Garmin for and still log them so MapMyRide is still the only logger that records everything I do. As I've used it so much I've taken a paid membership for this year, I have a policy of if something is properly useful to me and I can pay for it, I do.
When I got the Garmin I decided to also give Strava a go. It's just as easy as MapMyRide to log workouts on from the Garmin and keeps records just the same as MapMyRide it's got a lot more people on it so you get more of a feel for how you are doing on various segments, but equally it makes it a lot less likely that you are going to be up the top anywhere unless you are really good. I've no idea how to create routes on Strava or how to log workouts that haven't been automatically recorded but I don't really care as MapMyRide does that perfectly well for me.
My habit now is to finish my ride, plug the Garmin in to the PC, upload to MapMyRide and have a look at what I did, switch to Strava, upload to there, it's not really any hassle. While I prefer how MapMyRide presents most things, the analysis on Strava is better so it was much easier to see what speed I was doing at a specific bit of my ride on Strava when I really wanted to know it.