Inglourious Basterds came off, to me, as Tarantino's weakest project (still decent when all's said and done but very sloppy in my opinion). I went into Django Unchained with low expectations and it ended up being one of the best films I've seen in recent memory.
DU, despite being 15 minutes longer than IB, was tighter. It was nearly 3 hours long, but aside from shaving a minute off of the gratuitous slave-fighting scene there was nothing I would have cut. IB was shorter but I could have easily cut out a half-hour.
DU involves the characters doing things. IB involves the characters talking about doing things.
All of DU's main characters (Candie, Stephen, Brunhilda, Django and Schultz) all felt necessary to the narrative. Zoller in IB, despite taking up a fairly large portion of screen was more of an annoyance than a character. Shoshanna's lover Marcel doesn't have much character development.
Both films have a violent twist, but DU does it at a more unexpected moment during the contract signing rather than apeing off of Reservoir Dogs and saying "Let's kill everybody because we can".