Both are good for FrLg playthrough, considering there are no dark types. Let's have a comparison between them.
Alakazam
- 135 SpA and 120 Spe, which is humongous ingame.
- Availability of fully evolved form as soon as lvl 16 (Kadabra is already powerful in lvl 16)
- Play role as a sweeper.
- Frail, have less defense, okay ish special defense
- Requires catching Abra in Route 24/25 or 180 coins in Celadon Game Corner (120 in Leaf Green).
- Poor moveset, with availability of only Psychic/Psybeam and Shock Wave. I really hate Shadow Ball is physical in this gen... however I guess it doesn't matter as Gengar is the only dangerous Ghost here, and Alakazam can beat it through Psychic.
- Takes care of almost whole of Team Rocket and their grunts, Koga, Agatha (she should be a poison type specialist) and Bruno.
Mr. Mime
- 100 SpA and 90 Spe (good enough for this playthrough)
- 120 SpD. Acts as a special tank.
- Available around the same time as Alakazam (requires Abra trade). So it is also a pretty solid fully evolved Pokémon available very early.
- Less physical defense (Same as Alakazam).
- Moveset only slightly better than Alakazam (gets Thunderbolt and Magical Leaf). It can also be a Calm Mind user. It can act as an alternative for an electric Pokémon.
- Hits less harder than Alakazam, which might be crucial for some battles, where opponents are physical.
To sum up, Alakazam can hit harder but has lesser movepool, which is guess doesnt matter much. Mr. Mime can tank special hits and cover water types easily, hits less hard. Both are good, but I personally prefer Alakazam...
Alakazam
- Psybeam / Flash
- Psychic
- Shock Wave
- Calm Mind
Mr. Mime
- Psychic
- Thunderbolt
- Magical Leaf
- Calm Mind
Hope it helps... :)
Movesets can be edited...