let me sum up the differences between them.
Straight arm forehand = More power potential, Harder to time.
Double Bend forehand = Less power potential, Easier to time.
Because straight arm forehand have a bigger swing path making it easier to plow through the ball, but it's harder to time because you have to hit it out in front of you more which requires better footwork, anticipation, and ball judgement to hit it consistently.
Both forehands can be developed into world class forehands, double bend (in most cases) being the easier of the two.