I'm a huge proponent for Meganium, ask anyone. Problem is you're making Torterra out to be a piece of crap compared to Meganium, which is simply not true. In general terms, Torterra does have more uses than Meganium (Torterra: phys wall/phys tank/phys sweeper/subSeeder. Meganium: Subseeder/phys wall/sp. wall).
He also has a secondary type that happens to be one of (if not THE) best offensive types in the game.
Meganium, on the other hand, has better overall defenses (though he's bulkier physically, she can take both physical and special hits, which he can't) and doesn't have any crippling 4x weakness.
It's not as simple as saying one is better than the other, period. You have to look at both Pokemon, what they're capable of and what they're weak to before you decide which one is more suited to your style. Then you can give an informed opinion.