Jellicent
- Pretty specially bulky
- Great Abilities in Cursed Body and Water Absorb
- Three immunities with Water Absorb
- Decent Special Attack
- Learns moves like Surf, Waterfall, Psychic, Shadow Ball, Energy Ball and Scald
- Frillish evolves kind of late
- It's not very fast
Swanna
- Faster than Jellicent
- Gets a Ground immunity
- You can get Ducklett just in time for Clay
- Learns moves like Surf, Fly, Roost, Ice Beam, and Scald
- Weak in all other stats
- 4x weakness to Electric
- Learns nothing but Water and Flying moves in the level up pool
I would only say use Swanna if you want Fly. Otherwise, use Jellicent: it's bulkier, it has a better typing, better Abilities, better movepool, and higher stats that matter.
Jellicent @ Leftovers / Mystic Water
Ability: Water Absorb
- Surf
- Shadow Ball
- Sludge Wave / Ice Beam
- Energy Ball / Psychic / Waterfall
Hope I helped!