General

LEt me re-phrase my sentence better, which is better generally against every pokemon, wether it be uber ,ou ,uu ect

whichever has the higher speed stat.
Since shadow ball on either of them knocks out the other due to super-effective [for online battles lv50].

Plus with Mewtwo being available,
it's easier to go for Gengar since it's a ghost type instead choosing another psychic.

In terms of utility Gengar has a better move pool compared to Alakazam.

Mega Alakazam has a hire Sp.Atk and Speed, but Mega Gengar does have a bit more HP.a
Mega Alakazam does beat Mega Gengar in speed by a lot.

I will say who attacks first, seeing that Mega Alakazam speed is higher so it can attack first it will win.
Even if Mega Gengar does use one turn to use Trick Room Mega Alakazam would have attacked it!

Or Mega Gengar can use explosion and kill both.