I find it difficult to compare lenses of various focal lengths, since I usually use them for different purposes. If I compare the DA40, F50/1.7 and DA70, all of which I own and could be used for portraits, I would say that they are very close. Wide open the DA40 would be sharpest I think, but then again it is the slowest in the group. The DA70 seems to benefit more from being stopped down, so I can fool myself thinking that it is unsharp at F2.4 since it will be even better at F3.5 or so
I would say that my F50/1.7 is sharper at F2.4 then my DA70 at F2.4... then again, wide open vs stopped down.. difficult to compare like I said =)
I might suspect that my DA70 copy is just a tad soft though (compared to the praise it gets here on the forum), but I still love it