I would choose Crobat.
First of all, Crobat has superior base stats. It totals up to 535, while Swellow only has a total base stat of 430. Crobat also has better Attack and Speed. Crobat has 90/135 and Swellow has 85/125. Not only that, but also Crobat has better overall stats. Its Defenses are not bad, while Swellow's Defenses are like wet toilet paper. Swellow also has a somewhat shallow movepool, while Crobat has more options.
Here is a set for Crobat:
Note that I don't know if items/abilities existed back then, but I still put it.
Crobat@Life Orb/Choice Band/Leftovers
Ability: Inner Focus
EVs: 252 Atk/252 Spd/4 HP
- Fly/Aerial Ace
- Steel Wing
- Poison Fang
- (filler; suggestions: Double Edge, Confuse Ray, Pursuit)
Hope I helped!