Both are worth seeing. I really liked District 9, but I loved Inglourious Basterds, possibly the best movie of 2009. But not everyone likes Tarantino, so YMMV.
If you're in the mood for a smart SF/action movie, see District 9.
Inglourious Basterds has very little action. It's very talky with occasional and brief violence. If you love well-written, well-acted dialogue, see Inglourious Basterds.