the higher SR declares who goes first, then players teleport alternatively (including the Eldar Avatar), and when a player declines he may not change his mind and teleport another formation during the turn.

roll a dice if SR are equal

SPECIAL RULE
1.4.2 Abilities Used At Start Of Turn/Action
Some units have special abilities that are specified as taking effect at the beginning of the turn. These effects are resolved before the Strategy roll. Similarly, abilities that are used at the start of an action can be taken before the Initiative roll is made or the action chosen. If both players have effects for the beginning of the
turn, take turns resolving them, alternating between players for each subsequent effect until all effects are resolved. The player with the higher strategy rating decides which player will go first.