I caught both Abra and Gastly in the wild during my recent playthrough of Crystal and Abra was definitely the MVP of the team. Though I like the design of the Gastly line the best and they are both good Pokemon, Abra and its evolutions are faster and stronger so that gives them some advantage in battle. They are both built to sweep, but the tiny bit of extra speed for Abra can make all the difference sometimes. I went through Morty's gym destroying everything in one hit including Haunters and Gengar with my Kadabra because it outsped them and could hit them extremely hard with confusion/ psybeam, which the gastly line is weak to due the poison secondary typing.
Both the Gastly and Abra line will do well against Team Rocket, Kadabra/Alakazam by hitting their poison types for super-effective damage and Haunter/ Gengar by resisting with its dual typing and immunity to explosion. Both of them have good matchups against Chucks fighting types, Koga and Janine + Erika's grass types which often rely on poison, but should beware of Karen's dark types. Overall, they both have very safe typings, with a teeny edge to the Abra line having a more level playing field against Will and Sabrina's own psychic types.
Gastly was to me the more helpful of the two for catching Pokemon, its even the pokemon who allowed me to catch Abra by stopping it from teleporting away. It has useful moves like mean look and hypnosis which can help you trap the legendary beasts too. However, Gastly doesn't have quite as many offensive options in this generation as Abra does even though Gastly has amazing attacking power too. You have to rely on more gimmicky moves like curse, confuse ray or the hypnosis dream eater combo that involve risk. It can be fun to use but if you don't want any mess and want something that is going to kill most enemies the moment that its unleashed then use Abra.
The Abra line as a psychic type learns a very powerful stab move with Psychic, but the Gastly line as a ghost type does not have the equivalent. Shadow Ball is not a good move to teach Gastly prior to gen 4 because all ghost moves were physical, and Gastly and Abra can only be effective as special attackers. Abra doesn't have this problem because the move psychic has always been a special move. Gastly can learn the attack psychic and do a lot of damage with it, but you would not acquire the tm to teach it this until after the elite four, whereas Kadabra learns it naturally and always will be the more effective user of psychic because of the same type as bonus.
Gastly is better early game and easier to evolve because he can immediately use some damage dealing moves and fend for itself. You are going to have to help Abra level up by either switching out for another Pokemon and letting it share the exp or buying tms for it since the only move it knows is teleport until it evolves into Kadabra. However, once you make that investment its the deadlier of the two.
I recommend buying tms because its already extremely strong even in its earliest stage and has an even larger movepool than Gastly to take advantage of, so no need to inconvenience yourself by waiting. Abra can use all of the elemental punches available in Goldenrod City department store giving it amazing coverage since these are all special moves in GSC that play to the Abra line's strength. It can do super-effective damage with fire punch against Jasmine, Pryce and Erika, thunder punch to hit Misty and the endless water Pokemon trainers encountered by surf, Ice punches for Lance and Clair's dragons and a combo of the three ripping most of Red's team + Psychic for Morty, Team Rocket, Bruno, Koga and Janine, while resisting Will and Sabrina. With just these tools Kadabra is going to tear through most of the game by himself. Mine was part of Pokemon league team and did as much work or more than my starter pokemon Feraligatr.
