It's a close call but I think Seismitoad wins due to the following factors:
- It can have a strong Physical, Special or Mixed moveset, making it that much harder for your opponent to play against, whereas Ludicolo has no Physical moveset to speak of.
- Stealth Rock, allows you to set up hazards that make oncoming sweeps easier.
- Has fewer weaknesses, and for the Grass weakness it does have, it usually has Sludge Wave to make up for it, and Grass types are relatively rare in competitive play anyway. Ludicolo on the other hand has many weaknesses, to some very common types, and no immunity.
- Even gets Grass Knot which can OHKO unsuspecting opposing Seismitoad in a mirror match.
- Grass is usually less effective in competitive due to how many Pokemon it hits super effectively, versus how many resist it, and so on. Ground is usually more reliable.
All in all, despite the slight edge Seismitoad seemingly has over Ludicolo, it comes down to player preference and team synergy. Seismotoad just has a wider movepool that allows it carry out potentially many roles, making it easier to tailor to your team's needs.