A lot more is riding on a movie, writing-wise. Movies have to deliver a complete, three-act, self-contained storyline with a satisfying ending. For a sequel, this is a lot harder to do twice, especially if you wrapped everything up the first time.
In that sense, a TV series can deliver more than a movie, because it has a lot more time to develop its characters. You can show them doing laundry or filing their taxes; not EVERY SINGLE moment has to be devoted to the plot.
BUT....when a movie takes that more compressed schedule and executes it flawlessly, the results are far better than a TV series. Sarah Connor Chronicles was great, but Terminator II was phenomenal.