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I love both, but Meganium just doesn't match up well against the majority of threats in its own game, particularly early on with the flying, bug and Gastly gyms. It does okay with reflect but it doesn't learn leech seed or any powders besides poison, so it sidesteps the niche most early grass types have. Its spamming move is either magical leaf (lol) or petal dance which comes with its own set of problems.
Serperior gets leech seed, coil and leaf blade pretty early, so it can set up and sweep while still providing good support with glare, screens etc. The gyms are also much more friendly to grass types in gen5.
