In praise of primes - which next?

They're both certainly good lenses, there's no doubt about that.  There are advantages and disadvantages to each, though, besides the price and aperture range.

The center sharpness of the FA43 is slightly better than the center sharpness of the DA40; however, the edges/borders of the DA40 are much better (particularly at wide apertures).  Depending on what you're shooting, that may or may not matter much.  The differences in central sharpness are pretty much unnoticeable in actual everyday use.

The FA43 is also a little low in contrast wide open, while the DA40 has great contrast throughout its aperture range.  (Of course, wide open for the FA43 is beyond the aperture range of the DA40, so it's perhaps unfair to complain that it's not superior at that setting.)

The DA40 is noticeably faster in AF operations.  The FA43 is noticeably nicer to use for manual focus.

If one uses a film camera from time to time, the FA43 is obviously more appropriate; but holding out for a full-frame digital camera from Pentax anytime in the somewhat near term seems to me to be foolish.  They've made no announcements suggesting that they have one on the way, and no recent lens introductions have suggested anything other than APS-C and 645 in the future.  (At the very least, they'd presumably need to introduce a kit lens of some sort, a FF normalish zoom.  There's absolutely nothing like that in the lineup or in any lens roadmap I've seen.)

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--DrewE