At the same time, consider that the first-turn fighter gives you an “early” defensive bonus.
But the Allies do not need an early defensive purchase. If played right the Germans should not be able to take Karelia until turn 5 or 6 at the earliest, and this is without fighter purchases. So considering this longterm reality the shorterm is of no consequence. In my experience the Allies win by limiting/reducing Germany’s ipc level and then reducing their income. With this in mind I’d say a bomber or a carrier is always a better purchase because of their obvious uses.