Abra is the better choice. It evolves into Alakazam, the strongest Psychic Pokemon in the game, aside from Mewtwo. High Special and high Speed make it likely to hit first, and hit hard, especially with moves like Psychic and Ice Punch (provided you can trade it to Gold/Silver/Crystal and buy the TM).
Gastly is part Poison-type, which renders it weak against Psychic and Ground-type Pokemon. Lots of Pokemon learn either Psychic, Earthquake, or Dig, and with Gastly's low Defense, it practically beckons a Ground-type pwning. Doesn't do too well against Alakazam, either. Not much does.
By all means, go with the Disc One Nuke known as Abra.
gastly because certian types of moves dont do anything to it
Abra, because in my opinion he evolves into a stronger final stage as an alahkazam. just my opinion