It seems that the old F*/FA* should be a better lens optically, as the resolution difference is not as large as that of the DA40 and DA70 for what we could find when the last times Klaus re-tested the DA40 and 70 (here and here):-
http://www.photozone.de/pentax/131-pentax-smc-fa-300mm-f45-ed-if-review--lab-test-report?start=1
(Lab results of previous PZ FA*300 test on a K10D)
Also, the F/FA*300 is virtually 100% distortion free, which is close to perfect in this aspect. More importantly, with a more compact size and less weight, I would undoubtedly to choose the F*/FA* over DA if I had to choose. The less than a half-stop faster in the maximum lens speed is something that I can give up for all the advantages mentioned above. Nevertheless, I had my F*300 for long anyway.