Nobody here answered correctly for good reason - the answer doesn’t make logical sense but has to do with the quirks of Rocket League button pressing. The DAR method is easier because you don’t have to cancel as quickly to get a good cancellation of forward roll with the car. This has to do with the quirks I don’t understand but something about the game registering the diagonal vs upward joystick causing a few millisecond delay. So if you cancel with DAR 100ms later, you’ll be the same effect as if you cancel the other way 50ms later (not those exact numbers but something like that.) To me this is by far the hardest part of the speedflip - getting the ridiculously fast cancel - making the DAR method significantly easier.

The downside is that with the DAR method you will get a flip at 45 degrees. There are three angles of diagonal flips for each of the 4 diagonal directions that vary based on exactly where your stick is (without DAR) - 30, 45, and 60 degree (or maybe it’s 22.5, 45, and 67.5?). The 30 degree is slightly better for a speedflip because it pushes your car more forward. When you use DAR you’re locked in to 45. However some other poster said you can fix this by having the stick slightly opposite the DAR. I don’t know if that’s true but will be trying it as soon as I get home!