Since KEH has the "D" version pretty cheap, I was thinking of picking one up. I already have the "G" version and the build quality is very weak. I understand that the "D" is not pro build either, however, which one has the better build? I am looking for something at least comparible to my 50 F/1.8 D. Also, are there significant optical differences between the D & G? Your feedback is greatly appreciated.