As I have learned how to fly, I have found that one surface is great while the other one is not so great. But, the not so great surface for learning how to hover is perfect for learning forward flight.
When you are first starting out learning how to fly, you will be doing a lot of scooting and or hovering just a little off the ground. For this reason, concrete or asphalt is the best surface you can learn on. It will allow you to slide the helicopter around and learn how it's going to respond when you are on training gear. I tried to learn on grass, but the training gear would catch in it and I almost tipped it over a few times. So, I do not suggest starting out on grass.
Once you get to where you can take off, hover, and land pretty smoothly, then you are ready for forward flight. This is when you need to move over grass. Once you start moving the helicopter around, you are more likely to crash. If you crash on grass, you may save the heli from as much damage as crashing on a harder surface. So start out on Concrete or Asphalt if you can. You will thank me later.
