SWT and JFace go together, like the AWT and Swing go together. Since JFace is basically a set of SWT add-ons, it's not really meaningful to talk about it as an alternative to SWT.
So the question is whether to go with SWT or Swing. There have been endless arguments and discussions over this, but frankly, at the end of the day, there's very little difference. Given knowledgeable programmers to write the code on both sides, SWT+JFace isn't actually any faster than AWT+Swing. It doesn't actually look any better. It isn't any more or less portable.
I'd say Swing has a few (small) advantages: 1st, the pool of programmers is still larger, and 2nd, there are a few places -- like applets, for example -- where you just can't use SWT.