It's obvious he talks about mods, try searching before you post pointless questions (that is what I did).
I don't know about compability between the mods, but since Aether is only compatible with 1.7 and the Twilight Forest with 1.8, I don't think you can run both atm.
Also I don't know how much sprites TF adds, Aether adds a lot and there could be a possible sprite overload with both mods. And for the massive amount of new items, there could be conflicts with item IDs.
Edit: When I have to choose one of them, it would be Aether. Really hoping for compatibility in the future, though...
I've tried them both. The Aether is much more accomplished and better overall, with it's amazing mix of vanilla and something new. That's not to say that the Twilight Forest isn't good, as it introduces more adventure and exploration than vanilla does. Bottom line: both should be tried out, but the Aether is still better.