I would agree if 3D World had online.
However, one thing I don't like about it (and Mario games in general lately) is that it doesn't feel like an adventure, and a lot of the levels lack real theming. There are a lot of levels that are just random, and the aesthetics are just boring with random blocks. Some levels in the game are truly beautiful, but not as consistently as Lost World's levels. They are fun, but honestly, I doubt I will replay this game too often (except to beat each level with all 5 characters). Everything is just there.
So I think 3D World, so far, is the more polished and "safer" game. And it did surprise me with it's fun factor. But it feels...shallow. I was partly enjoying it so much because I was playing 3-player co-op, which is a blast. I probably wouldn't enjoy it as much in single-player, though.
Lost World, however, is just more innovative, and a more rewarding game, because of it's complex controls. And it feels like a true adventure, with much more open-ended levels. Granted, it requires time and practice, but I do appreciate games that are deeper in mechanics. 3D World is only as fun as it is because it has 4-player co-op, and it manages to throw something different at you each level. Lost World does the latter anyway (though 3D World is a bit more consistent in the quality of it's level gimmicks. I found Lost World's more interesting, though).
And to be honest, this has always been the key difference between every Mario and Sonic parallel game (release wise) - Mario is a more guaranteed good game, but not guaranteed to be stellar, and it can risk becoming stale (Super Mario 3D Land, and the NSMB series). When they are good, they are REALLY good, though (Galaxy 2 and 3D World). This is because Nintendo found an extremely simple and shallow formula that works, and don't change it that often. This is why the quality of Mario games, to me, is defined solely by level design and aesthetics. Because the game mechanics are going to be fine, but on their own, they are a bit boring. Even when you compare Super Mario Bros to Sonic 1, where the controls are almost identical, Sonic 1 is way more fun and has aged way better because it had the physics, slopes, and more open-ended levels that made it a deeper game than it seems at first glance.
Sonic is a much more innovative franchise, that never sticks to the same formula for long. Even when they find a winning formula (Sonic Rush, Sonic Unleashed), they never keep it for too long, and even when they develop these formulas, they add something significant each game (Wisps in Colours, open-ended levels and the return of Classic Sonic in Generations). While this was a barrier to the franchise's quality for a while in the past, it's been an asset for the past few years. Not to mention that this even extends to writers, voice actors, art styles, and music. While Sonic games can be terrible at times, the franchise NEVER gets old. And the games generally have a story and what not and good characterisation that ties things together, and make the games feel like adventures.
Basically, if you just want something you will most definitely enjoy, and are an impatient gamer, get 3D World. It's just a really fun game - however, it's mechanics are not very deep, so you won't really experiment much. You'll just blast through the game, and the levels are pretty linear most of the time. This makes it less fulfilling as a single-player game, but ideal as a multiplayer game
If you are willing to spend time with a game to experiment with it's mechanics and explore it, and are more likely to replay, despite it's few flaws, get Lost World. I've played it many a time after 100%ing it. This is a common trend with Sonic games, that I've noticed. However, I don't really do so with Mario games, and I definitely won't with 3D World (except to play multiplayer).
I don't think either game is better than the other. You just can't judge them on the same terms. They are meant to provide different experiences altogether. If anything, I'd say get BOTH. Than you have all bases covered exceptionally well!
After that, though, you kind of get used to it as you go on, since the gameplay style and scares are, for the most part, always the same, with the exception of item locations and certain puzzles. I always dread seeing Chris Walker (a giant fatass who has a nasty fetish for ripping your right arm off) though, since his attacks are more powerful than most of the other enemies', and he is also quite fast.