It totally depends.
We could help you better if you were to tell us the rest of your (future) team, but I'll lay down the facts for both:
Taillow --> Swellow:
- High speed
- Decent Attack
- Can learn Fly
- Average offensive movepool
- 3 weaknesses, 2 resistances, and 2 immunities
- Can not survive hits, but generally out-speeds and KOs the opponent before getting hit.
- Evolves at level 22
Wingull --> Pelipper:
- High Defense
- Great Special Defense
- Decent Special Attack
- Can learn Fly and Surf
- Good defensive movepool.
- 2 weaknesses (1 of them, Electric, is 4x, though, so keep that in mind), 5 resistances, and 1 immunity
- Can take a hit (not from electric moves, but generally otherwise it is somewhat bulkier)
- Evolves at level 25
In conclusion, it all boils down to this:
- Do you want a Special attacker or a Physical one?
- Do you have another Pokemon for Surf? (In which case, choose Swellow)
Also, note that while Pelipper has slightly better movesets, it have poor speed, and is more defensive/ support based.
Swellow is an all-out attacker.