I just checked the A20 rule book (pg 13 and 16), which has movement process and examples of movement. Both the intro and the examples have the player choosing a specific speed value number for the plane from the range of options on its card at the beginning of its turn. I hope this is helpful!
Anecdotal: For what it's worth, my gaming group (primarily new players) has occasionally house-ruled saying "normal/high" and then made a valid move within the range of options (without declaring "speed 3"). We play with 4-16 players each controlling a plane across 4 maps, so this keeps the game moving and prevents analysis paralysis.
The only in-game difference I can tell between the two methods are the potential consequences for failing a maneuver. The rule book method may leave a plane quite exposed, while the normal/high is more forgiving.
You roll for initiative to determine who has to move first. Then we place a facedown counter next to each of our planes.(either hi speed or low speed) Then we only reveal the token right before the plane moves. At that point you declare how many spaces it’s moving in according to its low or hi speed rating.
We found it gave a little bit more depth to the game without over complicating it.