LBA leads me to have both. I got the 77mm first and when I got my K-x with 40mm pancake, I couldn't resist the temptation of 70mm in small form factor to go with K-x.
Now I am torn with two outstanding lens that are only used infrequently. I wish I am a photog who can justify keeping both. They are precious prime lens for portraits.
For best value with cheaper cost, the 70mm is a no brain-er to get. If you are tight in budget and that you have the K-x, get the 70mm as f/2.4 is plenty to work with the higher ISO in K-x. If you have the K20D/K7D and that you wants the best in color, get the 77mm. Sharpness to me are outstanding in both of them. I have yet to use more to tell one over the other. In terms of color and contrast, the 77mm wins and it has that magical color rendition in both outdoor and indoor. AF is better in the 70mm and the quick shift focus in the 70mm is a handy plus over the 77mm especially in tricky scene needing manual control.
Thanks,
Hin