I ride about the same mileage each week, but not quite as fast as you! I would strongly suggest using the numbers your HRM spits out over anything on Strava or Fitday. As far as recording on Fitday, you kind of have two options: 1.) you can log time and miles and pay no attention to the calories (since you already use Strava it might be best to go with the next option), or 2.) you can make the activity type/time match what your HRM says you burned. So, for example you could enter in a lower speed from the drop down or use the 20+ mph, but less time so that the number of calories burned is more realistic.
I kind of do a combination of the two. My average is about 17mph when I'm out on my own, so I enter it as "12-13mph, leisure" (so about 1-2 "levels" below my actual activity) to get the calories more accurate. Then I record the actual number of miles and the time it took. That way the calorie burn FD reports is sort of in the ballpark.
It really depends on what your goals are. If you're trying to drop a bunch of weight before race season, logging calories as accurately as possible on FD is your best bet. Heck, you could even use a completely different activity (like karate or square dancing), just so your calorie burn is accurate. If (like me) you are more in a maintenance phase you probably shouldn't worry if the count is off a bit and just focus on logging miles and time. At best FD is just an estimation of what you actually did. Use your other tools to make the math work out. Also, when in doubt underestimate your activity (so in this case go with the numbers Strava gives) and overestimate your food intake -- if you're trying to drop a few pounds off your bike for cheap, that is the best way!!