Except the Wizard deciding to cast also has situational awareness so pre-declared spellcasting has it issues as well.
I know you have your debate going with Visionstorm. My solution to this point is to allow player to hold their character actions until the later in the turn. They can interrupt anybody moving after them and then take their action. This preserve the situational awareness of every character involved however the players who "wins" initiative has an advantage.
The issue with this is that it still assumes that the characters were aware that the wizard was casting a spell, and that that spell was specifically an aggressive area spell that could be avoided and not something else, which is not necessarily an automatic thing. The wizard could have been casting while blocked from view behind the corner of a wall when the characters started charging into melee, or he could've been casting a different spell. And even if the characters could see him and had time (and the knowledge) to figure out specifically what the wizard was doing then BOTH could have seen him and benefited from this--not just the character with the highest initiative, cuz "initiative". But that would have been a different scenario than the one I originally laid out, where the characters (both of them) managed to spot the wizard begin to cast a spell during the declaration phase and choose to do something different rather than charge into melee, which was the original plan.
But if they both opted to avoid the blast radius then WTF did they go do instead? Cuz there's NO scenario where either of them could have both: 1) charged into melee, which is also the blast point where the fireball takes place, and 2) also managed to avoid the 20 foot radius explosion that happens in that same spot around that time.
None of this was addressed in nDervish's post. It is simply assumed that the character with the highest initiative must have avoided the spell, cuz initiative, and that the character with the lowest initiative must have taken the blast, also cuz initiative. No mention of how that came to be or why the character with the lowest initiative kept changing into the blast zone when they saw the other character move way from the area or where. It is simply "initiative" therefore this character has an extra 20 feet of movement but the other character doesn't.