The thing is, both franchises soiled themselves before Hollywood got at 'em. Twilight wasn't that good (putting it nicely) to begin with, and then the whole thing went completely off the rails in Breaking Dawn. Eragon was predictable, but enjoyable, Eldest was a bit heavy handed and the details of the plot could be weird, but it was a definite page turner (Roran = awesome), and Brisingr was just embarrassing.
I'd say the Eragon movie was worse than the Twilight one, though, in terms of adaptation, because some things in the Twilight movie were actually better than the book (for example, Bella finds out that Edward's a vampire relatively quickly, not after 300 fricking pages of melodrama). And the baseball scene actually explained why they could only play in thunderstorms - it was a bit of an odd non-sequiter in the book.
Also, Twilight was more successful, both monetarily and critically (which isn't saying much), and in terms of a franchise's "health", it's probably helped it along - they're already making the next movie, and I think signed for the third, and it's raising awareness of the series. The Eragon movie? Not so much.
